Stats #24: What do all these numbers mean? Likelihood Ratios (September 10, 2003)

Stats #24: What Do All These Numbers Mean? Likelihood ratios

Content: This training class will teach you some of the numbers used to describe medical diagnostic tests. This class is useful for anyone who reads journal articles that evaluate these tests.

Objectives: In this class you will learn how to:

Calculate the likelihood ratio for a diagnostic test
Estimate pre-test probability and odds
Calculate post-test probabilities
Show how changes in prevalence affect the positive predictive value

Teaching strategies: Didactic lectures and small group exercises.

IRB Education Credits: This class does not qualify for IRB Education Credits (IRBECs).

Notes: Please bring a pocket calculator. I will use the following papers in class as examples:

Accuracy of a Single Question in Screening for Depression in a Cohort of Patients After Stroke: Comparative Study. Caroline Watkins, Leanne Daniels, Cathy Jack, Hazel Dickinson, and Martin van den Broek. BMJ 2001; 323: 1159. [Full text] [PDF]
The SCOFF Questionnaire and Clinical Interview for Eating Disorders in General Practice: Comparative Study. Amy J Luck, John F Morgan, Fiona Reid, Aileen O'Brien, Joan Brunton, Clare Price, Lin Perry, and J Hubert Lacey. BMJ 2002; 325: 755-756. [Full text] [PDF]
A clinical score to reduce unnecessary antibiotic use in patients with sore throat. W. J. McIsaac, D. White, D. Tannenbaum, D. E. Low. Cmaj 1998: 158(1); 75-83. [Medline] [Abstract] [PDF]

For the extended version of this class, I will also use the following page of recent weblog entries about diagnostic testing:

Stats #21/#24: Additional readings

Outline:

Overview of the STATS web pages
Consulting services that I provide
Exercises for Stats #24
Sensitivity
Specificity
Positive Predictive Value
Negative Predictive Value
Likelihood ratio
Fagan nomogram
Likelihood ratio slide rule
Please fill out an evaluation form

Overview of the STATS web pages (January 21, 2000)

What are the STATS web pages?

The STATS pages are a collection of handouts that I use in my job as a statistical consultant. The web provides a nice home for these handouts, because as I update my material, the newest version is immediately available to anyone who is interested.

Where can I find STATS?

If you have a web browser, like Internet Explorer or Netscape Navigator, you can surf on over to my site,

http://www.childrensmercy.org/stats

which is also found at http://internet1/stats, if you are attached to the Children's Mercy Hospital network. There are two obsolete sites: http://www.cmh.edu/stats and http://simon/stats. Do not use either of these sites.

Some of the fun stuff you can find on the STATS web pages.

Ask Professor Mean. For the tough Statistics questions that Dear Abby won't touch.

Planning Your Research Study. Things you need to plan for before you start collecting your data.

Selecting An Appropriate Sample Size. How much data do you really need?

Managing Your Research Data. Everything you want to know before you step to the keyboard.

Steps In a Typical Data Analysis. I have my data on the computer. Now what?

How to Read a Medical Journal Article. Reading a journal is hard work. Here's some help.

Professor Mean's Library. Good books and good web sites about Statistics.

... and even more good stuff!!!

This webpage was written by Steve Simon, edited by Linda Foland, and was last modified on 07/08/2008. Send feedback to ssimon at cmh dot edu or click on the email link at the top of the page. Category: Website details

For CMH employees only: Statistical Consulting Services.

You can get free statistical consulting if you work for Children's Mercy Hospital. Steve Simon and Ashley Sherman provide a wide range of statistical consulting services to help you with your research projects. This help can start as early as the initial planning of your research. I also help with the analysis of your data, using SPSS or other statistical software. We can also provide assistance with the preparation of your presentations and publications.

Here area some examples of the services that we have provided:

setting up your research hypothesis,

selecting and justifying your sample size,

writing the statistical methods section for your grant,

preparing randomization tables for your study,

reviewing your surveys for content and quality,

developing a system for entering your data,

choosing an appropriate statistical model for your data,

establishing validity and/or reliability for your measurement scales,

checking for violations of statistical assumptions in your data,

producing graphs and tables for your research publication, and

providing references for new and unusual statistical methods.

Specific statistical advice has been outlined on a series of web pages which can be found at http://www.childrensmercy.org/stats/. The pages provide advice about planning your research, selecting an appropriate sample size, managing your research data, performing a variety of data analyses, presenting research data, and writing research papers.

How to get in touch with a statistician

If you would like to meet with Steve Simon or Ashley Sherman, you can set up an appointment by emailing or calling Judy Champion (jmchampion (at) cmh (dot) edu or 816-983-6784). If you have a very simple question, send an email directly to us (ssimon (at) cmh (dot) edu and aksherman (at) cmh (dot) edu).

This webpage was written by Steve Simon on 2003-04-30, edited by Steve Simon, and was last modified on 2008-07-08. Send feedback to ssimon at cmh dot edu or click on the email link at the top of the page. Category: Professional details

Directions to my new office (April 25, 2008).

I have moved to a new office. It is a modular building just north of Children's Mercy Hospital. It is between 23rd and 22nd street, just off of Kenwood Avenue (Kenwood is a small north/south street just west of Holmes). If you need to get from your office to mine, here are some directions written by my Administrative Assistant, Judy Champion.

Take the elevator of the research tower down to the yellow level. Exit the employee parking garage on 23rd Street, walk to Kenwood and cross 23rd Street. Your destination is Building M 3 which is the building closest to 22nd Street. However, the entrance to our building faces Building M 2. It’s best to walk into the parking area that is just north of Building M 1 and follow the sidewalk around the west side of building M 2 in order to get to our building’s entrance on its south side. Another route would be to exit the Hospital Hill Center Building on Holmes and then walk ½ block north to 23rd Street, cross 23rd Street, walk west to Kenwood then north to building M 3 address 2220 Kenwood.

This webpage was written by Steve Simon and was last modified on 2008-07-14. Send feedback to ssimon at cmh dot edu or click on the email link at the top of the page. Category: Professional details

Stats >> Training >> Stats #24: Practice Exercises

1. Read the following abstract. The likelihood ratios for a PSA cutpoint of 4 ng/ml were deleted. Calculate these values using the remaining information in the abstract. Interpret these values.

Prostate-specific antigen testing accuracy in community practice. Hoffman RM, Gilliland FD, Adams-Cameron M, Hunt WC, Key CR. BMC Fam Pract 2002: 3(1); 19. [Medline] [Abstract] [Full text] [PDF]

BACKGROUND:
Most data on prostate-specific antigen (PSA) testing come from urologic cohorts comprised of volunteers for screening programs. We evaluated the diagnostic accuracy of PSA testing for detecting prostate cancer in community practice.
METHODS:
PSA testing results were compared with a reference standard of prostate biopsy. Subjects were 2,620 men 40 years and older undergoing (PSA) testing and biopsy from 1/1/95 through 12/31/98 in the Albuquerque, New Mexico metropolitan area. Diagnostic measures included the area under the receiver-operating characteristic curve, sensitivity, specificity, and likelihood ratios.
RESULTS:
Cancer was detected in 930 subjects (35%). The area under the ROC curve was 0.67 and the PSA cutpoint of 4 ng/ml had a sensitivity of 86% and a specificity of 33%. The likelihood ratio for a positive test (LR+) was [DELETED] and [DELETED] for a negative test (LR-). PSA testing was most sensitive (90%) but least specific (27%) in older men. Age-specific reference ranges improved specificity in older men (49%) but decreased sensitivity (70%), with an LR+ of 1.38. Lowering the PSA cutpoint to 2 ng/ml resulted in a sensitivity of 95%, a specificity of 20%, and an LR+ of 1.19.
CONCLUSIONS:
PSA testing had fair discriminating power for detecting prostate cancer in community practice. The PSA cutpoint of 4 ng/ml was sensitive but relatively non-specific and associated likelihood ratios only moderately revised probabilities for cancer. Using age-specific reference ranges and a PSA cutpoint below 4 ng/ml improved test specificity and sensitivity, respectively, but did not improve the overall accuracy of PSA testing.

2. Read the following abstract. The likelihood ratios for H. pylori culture were deleted. Calculate these values using the remaining information in the abstract. Interpret these values and the associated confidence intervals..

Laboratory diagnosis and susceptibility profile of Helicobacter pylori infection in the Philippines. Destura RV, Labio ED, Barrett LJ, Alcantara CS, Gloria VI, Daez ML, Guerrant RL. Ann Clin Microbiol Antimicrob 2004: 3(1); 25. [Medline] [Abstract] [Full text] [PDF]

BACKGROUND:
Helicobacter pylori diagnosis and susceptibility profile directs the applicability of recommended treatment regimens in our setting. To our knowledge, there is no published data on the culture and local susceptibility pattern of Helicobacter pylori in the Philippines.
METHODS:
52 dyspeptic adult patients undergoing endoscopy from the Outpatient Gastroenterology clinic of the University of the Philippines-Philippine General Hospital underwent multiple gastric biopsy and specimens were submitted for gram stain, culture, antimicrobial sensitivity testing, rapid urease test and histology. Antimicrobial susceptibility testing was done by Epsilometer testing (Etest) method against metronidazole, clarithromycin, amoxicillin, and tetracycline.
RESULTS:
Sixty percent (60%) of the study population was positive for H. pylori infection (mean age of 44 years +/- 13), 70% were males. H. pylori culture showed a sensitivity of 45% (95% CI [29.5-62.1]), specificity of 98% (95%CI [81.5-100%]), positive likelihood ratio of [DELETED] (95% CI [1.254-317.04]) and a negative likelihood ratio of [DELETED] (95% CI [0.406-0.772]). All H. pylori strains isolated were sensitive to metronidazole, clarithromycin, amoxicillin and tetracycline.
CONCLUSION:
Knowledge of the antibiotic susceptibility patterns in our setting allows us to be more cautious in the choice of first-line agents. Information on antibiotic susceptibility profile plays an important role in empiric antibiotic treatment and management of refractive cases.

What is sensitivity?

The sensitivity of a test is the probability that the test is positive when given to a group of patients with the disease. Sensitivity is sometimes abbreviated Sn.

The formula for sensitivity is

Sn = TP / (TP + FN)

where TP and FN are the number of true positive and false negative results, respectively. You can think of sensitivity as 1- the false negative rate. Notice that the denominator for sensitivity is the number of patients who have the disease. Using conditional probabilities, we can also define sensitivity as

Sn = P [ Test is positive | Patient has the disease ]

The following table summarizes these calculations.

A large sensitivity means that a negative test can rule out the disease. David Sackett coined the acronym "SnNOut" to help us remember this.

Here is an example of a sensitivity calculation.

In a study of 5,113 subjects checked for gastric cancer by endoscopy (Gut 1999; 44: 693-697), serum pepsinogen concentrations were also measured. A pepsinogen I concentration of less than 70 ng/ml and a ratio of pepsinogen I to pepsinogen II of less than 3 was considered a positive test. There were 13 patients with gastric cancer confirmed by endoscopy. 11 of these patients were positive on the test. The sensitivity is 11/13 = 85%.

This webpage was written by Steve Simon on 2005-08-18, edited by Steve Simon, and was last modified on 2008-07-08. This page needs minor revisions. Category: Definitions, Category: Diagnostic testing.

What is specificity?

The specificity of a test is the probability that the test will be negative among patients who do not have the disease. Specificity is sometimes abbreviated Sp. The formula for specificity is

Sp = TN / (TN + FP)

where TN and FP and the number of true negative and false positive results, respectively. You can think of specificity as 1 - the false positive rate. Notice that the denominator for specificity is the number of healthy patients. Using conditional probabilities, we can also define specificity as

Sp = P [ Test is negative | Patient is healthy ]

The following table summarizes these calculations.

A large specificity means that a positive test can rule in the disease. David Sackett coined the acronym "SpPIn" to help us remember this.

Here is an example of a specificity calculation.

In a study of the urine latex agglutination test (AJPH 1998;88(2):285-288), children were tested for H. influenzae using blood, urine, cerebrospinal fluid, or some combination of these. Of all the children tested, 1,352 did not have H. influenzae in any of these fluids. Only 9 of these patients tested positive on the urine latex agglutination test, the remaining 1,343 tested negative. The specificity is 1343 / 1352 = 99.3%.

What is a positive predictive value?

The positive predictive value of a test is the probability that the patient has the disease when restricted to those patients who test positive. This term is sometimes abbreviated as PPV. You can compute the positive predictive value as

PPV = TP / (TP + FP)

where TP and FP are the number of true positive and false positive results, respectively. Notice that the denominator for positive predictive value is the number of patients who test positive. You can also define the positive predictive value using conditional probabilities,

PPV = P [ Patient has the disease | Test is positive ].

If the prevalence of the disease in your situation is different from the prevalence of the disease in the research study you are examining, then you can use likelihood ratios to estimate the PPV.

The following table summarizes these calculations.

Do not calculate the positive predictive value on a sample where the prevalence of the disease was artificially controlled. For example, the PPV is meaningless in a study where you artificially recruited healthy and diseased patients in a one to one ratio.

Here is an example.

In a study of patients in a network of sentinel practices in Belgium ( BMJ 2000; 321; 998-999), 386 patients presented with rectal bleeding. These patients were followed from 18 to 30 months and 27 of them developed colorectal cancer. The positive predictive value for rectal bleeding is 27 / 386 = 7%.

What is a negative predictive value?

The negative predictive value of a test is the probability that the patient will not have the disease when restricted to all patients who test negative.

You can compute the negative predictive value as

NPV = TN / (TN + FN)

where TN and FN are the number of true negative and false negative results, respectively. Notice that the denominator for negative predictive value is the number of patients who test negative. You can also define the negative predictive value using conditional probabilities,

NPV = P [ Patient is healthy | Test is negative ].

If the prevalence of the disease in your situation is different from the prevalence of the disease in the research study you are examining, then you can use likelihood ratios to estimate the NPV.

The following table summarizes these calculations.

Do not calculate the negative predictive value on a sample where the prevalence of the disease was artificially controlled. For example, the NPV is meaningless in a study where you artificially recruited healthy and diseased patients in a one to one ratio.

Here is an example.

In a study of depression among 79 patients hospitalized for stroke (BMJ 2001; 323: 1159), 34 patients responded "no" to the question: Do you often feel sad or depressed? Among these 34 patients who tested negative, 6 had clinical depression as defined by a more complex measure, the Montgomery Asberg depression rating scale. Since 28 did not have depression, the negative predictive value is 28 / 34 = 82%. This sample is somewhat unusual in that the prevalence of depression is 43%. The authors recalculated negative predictive value for a range of different prevalences. If the group being examined would have had a prevalence of depression of only 10%, then the negative predictive value would have been 98%.

What is a likelihood ratio?

The likelihood ratio incorporates both the sensitivity and specificity of the test and provides a direct estimate of how much a test result will change the odds of having a disease. The likelihood ratio for a positive result (LR+) tells you how much the odds of the disease increase when a test is positive. The likelihood ratio for a negative result (LR-) tells you how much the odds of the disease decrease when a test is negative.

You combine the likelihood ratio with information about

the prevalence of the disease,

characteristics of your patient pool, and

information about this particular patient

to determine the post-test odds of disease.

If you want to quantify the effect of a diagnostic test, you have to first provide information about the patient. You need to specify the pre-test odds: the likelihood that the patient would have a specific disease prior to testing. The pre-test odds are usually related to the prevalence of the disease, though you might adjust it upwards or downwards depending on characteristics of your overall patient pool or of the individual patient.

You are probably more comfortable specifying a probability instead of an odds, and if so there are simple formulas for converting probabilities into odds. You also may have some uncertainty about the pre-test odds. In this case, you might propose a range of values that seem plausible.

You can summarize information about the diagnostic test itself using a measure called the likelihood ratio. The likelihood ratio combines information about the sensitivity and specificity. It tells you how much a positive or negative result changes the likelihood that a patient would have the disease.

The likelihood ratio of a positive test result (LR+) is sensitivity divided by 1- specificity.

The likelihood ratio of a negative test result (LR-) is 1- sensitivity divided by specificity.

Once you have specified the pre-test odds, you multiply them by the likelihood ratio. This gives you the post-test odds.

The post-test odds represent the chances that your patient has a disease. It incorporates information about the disease prevalence, the patient pool, and specific patient risk factors (pre-test odds) and information about the diagnostic test itself (the likelihood ratio).

Example

An early test for developmental dysplasia of the hip. The test has 92% sensitivity and 86% specificity in boys (AJPH 1998; 88(2): 285-288). The likelihood ratio for a positive result from this test is 0.92 / (1-0.86) = 6.6 for boys. The likelihood ratio for a negative result from this test is (1-0.92) / 0.86 = 0.09 (or roughly 1/11).

Suppose one of our patients is a boy with no special risk factors. The diagnostic test is positive. What can we say about the chances that this boy will develop hip dysplasia? The prevalence of this condition is 1.5% in boys. This corresponds to an odds of one to 66. Multiply the odds by the likelihood ratio, you get 6.6 to 66 or roughly 1 to 10. The post test odds of having the disease is 1 to 10 which corresponds to a probability of 9%.

Suppose we had a negative result, but it was with a boy who had a family history of hip dysplasia. Suppose the family history would change the pre-test probability to 25%. How likely is hip dysplasia, factoring in both the family history and the negative test result? A probability of 25% corresponds to an odds of 1 to 3. The likelihood ratio for a negative result is 0.09 or 1/11. So the post-test odds would be roughly 1 to 33, which corresponds to a probability of 3%.

Notice that a negative test seems to change things more than a positive test. There are two factors at work here. First, a positive result multiplies the pre-test odds by a factor of only seven whereas a negative result divides the pre-test odds by 11. This means that the test is better at ruling out a condition than ruling it in.

Second, the impact of a test is usually greatest for mid-sized probabilities. If a condition is either very rare, or very common, then only a very definitive test is likely to change things much. But mid-sized probabilities (say between 20% and 80%) will change greatly on the basis of even a moderately precise test.

Summary

The likelihood ratio, which combines information from sensitivity and specificity, gives an indication of how much the odds of disease change based on a positive or a negative result. You need to know the pre-test odds, which incorporates information about prevalence of the disease, characteristics of your patient pool, and specific information about this patient. You then multiply the pre-test odds by the likelihood ratio to get the post-test odds.

This webpage was written by Steve Simon on 2005-08-18, edited by Steve Simon, and was last modified on 2008-07-14. This page needs minor revisions. Category: Definitions, Category: Diagnostic testing.

What is a Fagan nomogram?

The Fagan nomogram is a graphical tool for estimating how much the result on a diagnostic test changes the probability that a patient has a disease (NEJM 1975; 293: 257). A picture of the Fagan nomogram appears below.

To use this tool, you need to provide your best estimate of the probability of the disease prior to testing. This is usually related to the prevalence of the disease, though this may be modified up or down on the basis of certain risk factors that are present in your patient pool or possibly in this particular patient. You also need to know the likelihood ratio for the diagnostic test.

With this information, draw a line connecting the pre-test probability and the likelihood ratio. Extend this line until it intersects with the post-test probability. The point of intersection is the new estimate of the probability that your patient has this disease.

More details

Here are details on how the graph works and how you could construct a similar graph yourself. The principle is very much similar to a slide rule.

First, the computations involved use odds rather than ratios. If you multiply the pre-test odds by the likelihood ratio, you will get the post-test odds. And since multiplication of two numbers is equivalent to adding their logarithms, we use a log scaling for both the odds and the likelihood ratio.

The official formula is:

and the Fagan nomogram uses the equivalent formula

So although the labels on the left and right are written in terms of probability, the tick marks are spaced at the log odds. For technical reasons.we have to set the scaling of the log likelihood ratio to 1/2 that of the log odds. We also have to invert the scale for the log pre-test odds.

So if you wanted to construct this graph yourself, simply plot a range of log odds at x=+1. Plot an inverted range of log odds at x=+1. Write labels in terms of probabilities rather than odds. Then plot 1/2 of the log likelihood ratio values at x=0.

Examplehttp://jama.ama-assn.org/cgi/reprint/291/24/2990.pdf

Here are a couple examples of how to use the Fagan nomogram.

A study of an early test for developmental dysplasia of the hip (AJPH 1998; 88(2): 285-288) computed a likelihood ratio for a positive result as 7 for boys (5 for girls). The prevalence of this condition is 1.5% in boys (6% for girls). Suppose one of our patients is a boy with no special risk factors. The diagnostic test is positive. What can we say about the chances that this boy will develop hip dysplasia?

The post-test probability is a bit below 10%.

Suppose this boy had a family history of hip dysplasia that would increase our pre-test probability to 25%. How much would our assessment change if we.had a negative test result?

The likelihood ratio for a negative result is 0.09 for boys (0.2 for girls) So we would draw a line connecting the pre-test probability of slightly more than 20% to a likelihood ratio about just a bit below .1

This leads to a post-test probability around 3%.

A web based version of the Fagan Nomogram is available at www.cebm.net/nomogram.asp. This version requires the Shockwave plug-in.

Summary

You can use a Fagan nomogram to calculate disease probabilities. You draw a line connecting the pre-test probability of disease and the likelihood ratio. When you extend this line to the right, it intersects at the post-test probability of disease.

Likelihood ratio slide rule (October 24, 2002) Category: Diagnostic testing

The use of likelihood ratios requires a bit of tedious calculations. I have developed a simple slide rule that will do likelihood ratio calculations for you.

Note: I am developing a special handout (PDF format) that explains the mathematics behind diagnostic testing and which illustrates many of the important points using the likelihood ratio slide rule. I distributed this handout in a talk for the American College of Allergy, Asthma & Immunology on Sunday, November 11, but ran out very quickly.

Assembly instructions

Please print out this graphic image of the likelihood ratio slide rule (PDF format). An earlier version of this slide rule is also available.

Cut out the bottom piece (the sleeve) and the top piece (the insert). Also cut out the two rectangles in the middle of the sleeve. Fold the left and right portions of the sleeve behind and tape them together. Double sided tape works very well for this. Slip the insert into the sleeve. You may need to trim a tiny amount off the left and right sides of the insert to get it to fit well. You want the insert to fit not too snugly and not too loosely inside the sleeve.

For a more durable slide rule

If you print this to a regular sheet of paper, the slide rule will be okay but a bit flimsy and easy to bend. For a more durable slide rule, print out the image on a thick piece of paper or tape/glue the image to a thin piece of cardboard. You can also print the image on a full sheet adhesive label (like Avery 5165) and then attach the label to a thick piece of paper or a thin piece of cardboard.

How to use the slide rule

Slide the insert up or down until the pre-test probability in the left window lines up with the likelihood ratio. Read the post-test probability in the right window.

Examples

In Watkins et al 2001, a single question diagnostic test (the Yale-Brown obsessive-compulsive scale) was compared to a "gold standard" measure of depression, the Montgomery Asberg depression rating scale (MADRS).

On the MADRS 43 (54%) were classified as clinically depressed; 37 answered "yes" to the Yale single question and six answered "no." Of the 36 classified as not depressed, eight answered "yes" and 28 "no." The values (95% confidence intervals) for the Yale test were sensitivity 86% (75% to 97%), specificity 78% (65% to 91%), positive predictive value 82% (71% to 93%), negative predictive value 82% (69% to 95%); 82% (73% to 91%) of cases were classified correctly.

The prevalence of depression in this population was unusually high, so the authors presented additional positive predictive values (PPV) and negative predictive values (NPV) for prevalence values ranging from 10% to 90%. An abridged version of their table appears below.

Prevalence PPV NPV

90% 97% 38%

80% 94% 58%

70% 90% 70%

60% 85% 79%

50% 80% 85%

40% 72% 89%

30% 63% 93%

20% 49% 96%

10% 30% 98%

Since the PPV is simply the post-test probability after a positive test, we can use the likelihood ratio slide rule to re-create their calculations. First, we need to compute the likelihood ratio for a positive test (LR+). The formula is

LR+ = Sn / (1-Sp) = 0.86 / (1-0.78) = 3.9

where Sn and Sp are sensitivity and specificity, respectively. We will round this value to 4.

To compute the positive predictive value when the prevalence of the disease is 10%, line up the 10% pre-test probability with the likelihood ratio of 4 (the unlabelled tick mark between 3 and 5). In the right side window, the post-test probability should be slightly more than 30%, which matches the value computed by Watkins.

Slide the insert up so the 20% pre-test probability lines up with the likelihood ratio of 4. The post-test probability should be around 50% which also matches the value in Watkins.

Now slide the insert up so the 30% pre-test probability lines up with the likelihood ratio of 4. The post-test probability should be slightly more than 60%.

Repeat this for 40%, through 90% and see if you can estimate the remaining PPV values.

To compute NPV, we need to calculate the likelihood ratio for a negative test (LR-). The formula is

LR- = (1-Sp) / Sn = (1-0.86) / 0.78 = 0.18.

There is no tick mark for 0.18, so we will use a point about halfway between the 0.15 and 0.2 tick marks. Line up the prevalence of 10% with the likelihood ratio of 0.18 and read off the post-test probability of 2% in the right side window. Since there is only a 2% chance of having the disease, there is a 98% of being healthy, which matches the NPV computed by Watkins.

Line up a prevalence of 20% with the likelihood ratio of 0.18 to get a post-test probability of 4% and an NPV of 96%.

Now line up a prevalence of 30% with the likelihood ratio of 0.18 to get a post-test probability of 7% and an NPV of 93%.

Repeat this for 40% through 90% and estimate the remaining NPV values.

Second example

A letter to the editor in BMJ commented on how the use of likelihood ratios could have simplified the interpretation of results of a rapid whole blood test for diagnosing Helicobacter pylori infection.

In that study the likelihood ratio for a positive test result was 9.8. The advantage of knowing this is that it can be applied to similar patients in other populations to estimate the predictive value of the test, provided that the pre-test probability of disease can be estimated. For example, H pylori is found in 48% of dyspeptic patients in the community (the pre-test probability), so therefore a positive rapid blood test with a likelihood ratio of 9.8 applied to this population would give a post-test probability (or predictive value) of 90% (this can be estimated using a simple calculation or a nomogram). --BMJ 1997; 314: 1688.

We have to round a bit here. Line up a pre-test probability of 50% with a likelihood ratio of 10. Read the post-test probability of slightly more than 90% in the upper window.

Third example

Buschbaum et al examined the sensitivity, specificity, and likelihood ratio for the CAGE score, a series of yes/no answers to four questions (Ann Intern Med 1991; 115(10): 774-777). The four item scale was very good at detecting alcohol abuse or dependence.

Score Abuse or
Dependence No abuse or
dependence Likelihood
ratio

0 33 428 0.14

1 45 54 1.5

2 86 34 4.5

3 74 10 13

4 56 1 100

In this paper, the authors noted a prevalence of alcohol abuse and dependence of 36%. Find this value in the pre-test probability and line it up successively with each of the likelihood ratios listed above. You should get a post-test probability of 7%, 45%, 70%, 90% and 98% for the scores of 0 through 4, which matches up nicely with the values given in the paper. The likelihood ratio slide rule computations are shown below for the first three of these cases.

Grant et al tabulated the prevalence of alcohol abuse or dependence for demographic groups. This rate varies by age (higher among younger people), by gender (higher among males) and race (higher among non-blacks). Among non-black males, for example, the prevalence is 23%, 11%, 6%, and 1% for 18-29, 30-44, 45-64, and 65+ years of age, respectively (Alcohol Health & Research World 1994; 18(3):243-248, as quoted in alcoholism.about.com/library/nabdep4.htm).

The prevalence would be roughly twice as high among ambulatory patients than the general population and four times as high for hospitalized patients than the general population (Postgraduate Medicine Online 1996; 100(1), www.postgradmed.com/issues/1996/07_96/blondell.htm).

Suppose you apply the CAGE score to a 70 year old hospitalized white male. This person scores 3 on CAGE. Line up a pre-test probability of 4% with a likelihood ratio of 13. The post test probability is slightly more than 30%.

Suppose you give the same test to a 35 year old white male who visits your clinic and he scores 0 on CAGE. Line up a pre-test probability of 22% with a likelihood ratio of 0.14. The post-test probability is 4%.

How does it work?

The likelihood ratio slide rule works on the same principle as a regular slide rule. The logarithms on a slide rule allow you to multiply simply by adding. It uses the simple formula

log (a*b) = log (a) + log (b).

There's an old joke well known among mathematicians about logarithms. After the flood waters receded, Noah commanded the animals to go forth and multiply. The snakes went up to Noah and told him they couldn't multiply because they were adders. So Noah built them a piece of wooden furniture with a flat top and four legs. The adders could now multiply because they had a log table.

The formula for computing post-test odds is

post-test odds = likelihood ratio * pre-test odds.

By taking logarithms of both sides of the equation, we get

log (post-test odds) = log (likelihood ratio) + log (pre-test odds)

Sliding the insert up or down will add a pre-test log odds value to a log likelihood ratio to get a post-test log odds value. The tick marks are labeled using probability rather than odds to simplify things further.

The likelihood ratio slide rule that I developed was inspired by the Fagan nomogram which also uses logarithms. In the Fagan nomogram, you draw a line connecting the pre-test probability with the likelihood ratio. Extend the line further to the right to compute the post-test probability.

Summary

The likelihood ratio slide rule allows you to compute the post-test probability of a disease given the pre-test probability and the likelihood ratio of a diagnostic test. Simply line up the pre-test probability in the left side window with the likelihood ratio. Then read the post-test probability in the right side window.

This webpage was written by Steve Simon and was last modified on 07/08/2008.

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