Asthma laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor(s)-In-Chief: Lakshmi Gopalakrishnan, M.B.B.S. [3]

Overview

Routine laboratory tests are not indicated for the diagnosis of asthma but may be used to exclude other causes of wheeze. Elevated eosinophil count and elevated serum IgE levels may be observed in patients with a repeated history of allergic trigger. Arterial blood gas and pulse oximetry may be used to assess the disease severity and response to therapy.

Serum and Sputum Examination

• A markedly elevated serum eosinophil count greater than 15% may suggest an underlying allergic disease.

• Charcot-Leyden crystals are microscopic eosinophilic lysophospholipase crystals that are present in the sputum and are indicative of an eosinophilic inflammation or proliferation, such as found in allergic reactions. Curschmann's spirals are mucus casts that may also be seen in the distal airways.

• Measurement of sputum eosinophil count has shown to predict the response to steroid therapy and also help to monitor the airway inflammation. A small controlled prospective study demonstrated a significant symptomatic benefit with the use of steroid to control sputum eosinophilia. Thereby, suggested normalisation of the induced sputum eosinophil count to reduce the incidence of asthma exacerbations and admissions and also reduced the need for additional anti-inflammatory therapy.

Arterial Blood Gas

• Arterial blood-gas is used to evaluate the respiratory function and may be reserved to patients with severe acute exacerbation of asthma at which point ABG may reveal respiratory alkalosis that is consistent with the hypoxemia and/or hypercarbia secondary to significant hypoventilation.

• Hypercarbia in particular is an important indicator of hypoventilation, that may be used to decide the need for mechanical ventilation, in cases of markedly elevated PCO2.

Pulse Oximetry

• Measurement of oxygen saturation by pulse oximetry may be useful to identify patients with acute severe asthma who may rapidly progress to respiratory failure and thereby require more intensive therapy.

• Pulse oximetry may be useful to exclude hypoxemia, particularly in uncomplicated cases, wherein administration of oxygen is useful to eliminate hypoxemia.