Asthma laboratory tests

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, which are commonly found in allergic reactions. Curschmann's spirals are mucus casts that may also be observed in the distal airways.


 * Measurement of sputum eosinophil count has been shown to predict the response to steroid therapy and also help to monitor airway inflammation. In a small controlled prospective study there was a demonstrated significant symptomatic benefit with the use of steroid when compared to the control sputum eosinophilia. This evidence demonstrated that the normalization of the induced sputum eosinophil count reduced the incidence of asthma exacerbations and hospital admissions and further reduced the need for additional anti-inflammatory therapy.

Arterial Blood Gas

 * Arterial blood-gas is used to evaluate the respiratory function. It may be reserved to patients with severe acute exacerbation of asthma. In severe acute exacerbation, 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. This, in turn, 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.

Allergy Testing
The two most commonly used tests to assess the allergic sensitivity to specific envirnomental allergens include allergic skin testing and measurement of serum IgE levels.

Serum IgE

 * A normal total serum IgE level does not exclude the diagnosis of asthma; however in a patient with recurrent episodes secondary to allergen exposure, may report positive for allergic tests and is associated with a higher positive predictive value. A total serum IgE level of greater than 100 IU/mL is characteristic of underlying allergic disease; however, levels greater than 1000 IU/mL is highly suggestive of eczema or allergic bronchopulmonary aspergillosis.


 * Allergen-specific IgE using immuno-fluorescence is most commonly employed to identify specific envirnomental allergens and have replaced radio-allergo-sorbent tests (RAST).

Allergy Skin Test

 * Skin testing is another method that may be used to assess the allergic sensitivity to specific aero-allergens such as dust, pollen and mold spores.


 * Asthmatics and patients with associated allergic rhinitis may benefit from allergy immunotherapy.