Diagnostic Testing and Pulmonary Function
Core concepts
Spirometry measures forced expiratory volume in 1 second, forced vital capacity, the ratio, and forced expiratory flow. It is effort-dependent and usually feasible from about age 5–6. An obstructive pattern shows a reduced forced expiratory volume in 1 second to forced vital capacity ratio. A restrictive pattern shows reduced forced vital capacity with a preserved or high ratio and should be confirmed with lung volumes. A bronchodilator response is generally a 12% or greater increase in forced expiratory volume in 1 second, interpreted in clinical context in children. Bronchoprovocation with methacholine or exercise can support an asthma diagnosis. Exhaled nitric oxide reflects eosinophilic inflammation. Diffusing capacity assesses gas transfer. Sweat chloride and CFTR genetic testing evaluate cystic fibrosis. Chest radiography and computed tomography assess structural disease. Arterial blood gas measures ventilation and oxygenation.
Traps
Do not diagnose restriction from spirometry alone. Do not overinterpret forced expiratory flow between 25% and 75% of vital capacity. Ignoring effort and reproducibility leads to errors. Adult reversibility criteria should not be applied rigidly to children. Normal spirometry between episodes does not exclude asthma.
Worked example and rule of thumb
A child has forced expiratory volume in 1 second 68% predicted, forced vital capacity 88% predicted, and ratio 0.77, rising to 82% after bronchodilator. This is obstructive and reversible. Rule of thumb: obstruction lowers the ratio; restriction lowers forced vital capacity with a normal or high ratio.