Diagnostic tests in asthma
| Risk of overdiagnosis | Risk of underdiagnosis | Notes | |
| Peak flow variability#,¶,+ | Patient may have variable technique and/or have inaccurately charted PEFR. | Poorly sensitive (3–46% for identifying physician diagnosed asthma) [5]. Less sensitive if carried out when asymptomatic. | This test is dependent on patient effort and technique and it is unobserved. Patients need to do peak flows twice daily for 2 weeks and accurately chart them. Peak flow charts are easily available to healthcare professionals and low cost. |
| Spirometry#,¶,+ | Obstructed spirometry may be due to other conditions (e.g. COPD, bronchiolitis obliterans). In the elderly a ratio of <0.7 can be normal. | Usually a ratio of <0.7 (FEV1/FVC) is interpreted as obstructive; however, in the young a normal FEV1/FVC ratio is significantly higher than this, leading to possible false negatives. Normal spirometry at a single time point does not rule out asthma. | Need to have reproducible results for this to be a reliable measurement and this is dependent on the operator and patient effort, and potentially on patient coughing etc. Widely available (but not universally in primary care). |
| Bronchodilator response#,¶,+ | Patients may have reversibility in other diseases (e.g. COPD). | Patients may have used a bronchodilator on the day of the test, or a long-acting one even 1–2 days before. If patients are well at the time of the test and have normal spirometry no response may be seen. In chronic asthma patients may develop fixed airflow obstruction. | Need to have reproducible results for this to be a reliable measurement and this is dependent on the operator plus patient effort, and potentially limited by patient coughing etc. Widely available (but not universally in primary care). |
| Measures of airway hyperresponsiveness Direct provocation (methacholine, histamine)#,¶,+ Indirect provocation (mannitol, hypertonic saline, exercise, eucapnic hyperventilation)#,¶ | A proportion of the normal, asymptomatic population will have a positive test. False positives more likely in COPD, cystic fibrosis, allergic rhinitis. | If patients are on asthma treatment the sensitivity of the test drops and a negative test does not rule out asthma. | Generally not available in primary care. Of less value if patient has established airflow obstruction. |
| FeNO+ | FeNO can be raised in other conditions (e.g. allergic rhinitis, eosinophilic bronchitis, COPD with an eosinophilic phenotype). | FeNO is suppressed in smokers. Some asthmatics will not have eosinophilic airway inflammation at the time of testing (e.g. neutrophilic/paucigranulocytic asthma). Asthma patients who are already on ICS treatment may have a normal FeNO. | Not consistently available in primary care. |
| Blood eosinophils | May be raised in numerous other conditions including COPD, allergic conditions, parasitic infections. | Treated asthmatics or those who are not currently exacerbating may have normal blood eosinophils. Does not help identify non-eosinophilic asthmatics. | Not a point of care test, so information not available to the treating clinician immediately. |
| Sputum eosinophils | May be raised in other conditions (e.g. COPD with an eosinophilic phenotype, eosinophilic bronchitis). | May be suppressed by treatment with ICS or OCS. | Only available in specialist centres, requires expertise and is expensive and time consuming. |
PEFR: peak expiratory flow rate; FEV1: forced expiratory volume in 1 s; FVC: forced vital capacity; OCS: oral corticosteroids. #: GINA guidelines; ¶: BTS/SIGN guidelines; +: NICE guidelines.