Blood tests

Many routine blood tests are performed when a patient has either an acute or a chronic respiratory disease. Some blood tests are done specifically to monitor drug levels (e.g. theophylline) or to confirm a diagnosis (e.g. specific IgE levels in inhalant allergy), but most are indicators of a disease process such as infection. The more common tests and the associated lung diseases are listed down.

1. White blood cell count (WBC) ⚠ Elevated in infections such as pneumonia 2. Haemaglobin (Hb) ⚠ Anaemia (low Hb) is often present in chronic pulmonary abscesses ⚠ Erythrocytosis (high red blood cell (RBC)) is sometimes seen in COPD and may indicate hypoxaemia 3. C-reactive protein (CRP) ⚠ An inflammatory index, which can be raised in adults with bacterial pneumonia, pulmonary abcess, infective exacerbations of COPD and asthma. Can be raised in idiopathic pulmonary fibrosis 4. Erythrocyte sedimentation rate (ESR) ⚠ Another inflammatory index, increased during infection. Can be raised in idiopathic pulmonary fibrosis 5. Blood cultures ⚠ Important to obtain during lung or other respiratory infection, prior to starting antibiotics 6. Serum IgE ⚠ Total serum IgE and specific IgE levels rise in allergic bronchopulmonary aspergillosis. ⚠ In asthma, specific IgE antibodies (radioallergosorbent tes [RAST] tests) are measured to confirm diagnosis of inhalant allergies. ⚠ Total IgE shows a positive correlation with prevalence of asthma, but is not diagnostic 7. Serum α1antitrypsin levels ⚠ Should be measured in younger patients with emphysema. If level is low, the phenotype or genotype should also be identified 8. Serum immunoglobulins and IgG subclasses ⚠ These tests should be performed in patients suspected of having bronchiactasis: as immunoglobulin deficiency is a relatively common cause 9. D-dimer ⚠ This test is an indirect, but suggestive marker of intravascular thrombosis. It has a low specificity and can be positive not only when there is pulmonary embolism or deep vein thrombosis, also in the presence of malignancy and after trauma