2. Asthma Diagnosis

The diagnosis of asthma is clinical. Asthma is defined by:

Chronic recurrent respiratory symptoms (coughing, wheezing, chest tightness, shortness of breath)
Variable airflow limitation (evidenced in spirometry or, peak flow) in children > 5 years of age
Reversibility of airflow limitation
Exclusion of alternate diagnoses
The following definitions of airflow obstruction based on spirometry are below:

AIRFLOW OBSTRUCTION (CHILDREN < 12 YEARS OLD)

FEV₁reduced to less than 80 percent predicted
FEV₁/FVS ratio of less than 0.85 (85 percent)
Daily diurnal PEF variability > 10%

AIRFLOW OBSTRUCTION (CHILDREN > 12 YEARS OLD)

FEV₁reduced to less than 80 percent predicted
FEV₁/FVC is reduced below normal - less than 0.70 or less than 5th percentile (lower limit of normal [LLN])
Daily diurnal PEF variability > 10%

REVERSIBILITY

In patients with baseline airflow limitation, an increase in FEV₁ and/or FVC > percent over baseline and > 200 mL after inhaled bronchodilator
In patients with normal spirometry, FEV₁ of more than 8 percent following inhaled bronchodilator
Consider reversibility based on improvement in clinical symptoms to therapies.

Asthma Predictive Index

The modified Asthma Predictive Index (API) is a useful tool to for health care professionals to predict which children may develop asthma in later childhood.

The API was designed to be used in children under the age of three years old.

Modified API is only applicable in children with 4 or more wheezing episodes. A positive modified API is when there are 1 or more major criteria OR 2 or more minor criteria in a child with 4 or more wheezing episodes. A positive modified APO is >98% specific for an asthma diagnosis by age 11.

Major Criteria

History of asthma in parent(s)
MD diagnosed eczema
Aeroallergen sensitization

Minor Criteria

Wheezing unrelated to colds
Eosinophils > 4% on CBC with diff
Food allergy to milk, egg, peanuts

References: Guilbert TW, et al. Long-term inhaled corticosteroids in preschool children at high risk for asthma. N Engl J Med. 2006 May 11;354(19):1985-97 Chang TS, Lemanske RF, Guilbert TW, et al. Evaluation of the Modified Asthma Predictive Index in High-Risk Preschool Children. The journal of allergy and clinical immunology in practice. 2013; 1(2):10.1016/j.jaip.2012.10.008. doi:10.1016/j.jaip.2012.10.008.

Methacholine Challenge Test

The methacholine challenge is a bronchoprovocation test.

Bronchoprovocation test

Useful when the diagnosis of asthma is in question

Historical definition of a positive methacholine challenge was when FEV₁decreases more than 20% with inhaled agent or the PC₂₀, the provocation concentration causing a 20% decline in FEV_1.

2017 European Respiratory Society Technical Standards recommended a shift from using 20% change to PD₂₀ the provocation dose resulting in a 20% decline FEV₁ to calculate level of airway hyperresponsiveness. See table below.

Sensitivity 100%, specificity near 50%

PD_{20 (Micrograms)}	Level of airway hyperresponsiveness
>400	Normal
100-400	Borderline
25-100	Mild
6-25	Moderate
<6	Marked