Introduction and Objective Our lung investigation unit introduced indirect bronchial provocation challenge tests with mannitol to replace direct bronchial provocation tests with Methacholine. Mannitol challenge tests have practical and safety profile advantages. The sensitivity and specificity for PC20 Methacholine are 91% (84.2%–97.8%) and 90% (76.9%–100%) respectively.1 The specificity of PD15 Mannitol compares well to Methacholine at 98.4% (96.2%–99.4%), but the sensitivity of PD15 Mannitol is lower at 58.8% (50.7%–62.6%).2 The aim of the study was to review the clinical interpretation of mannitol challenge test results in the diagnosis of asthma.
Methods Data were collected on all Mannitol challenge tests performed between July 2008 and January 2011. A retrospective analysis of case notes was performed to assess the indication for the test, the interpretation of results and any subsequent changes in management.
Results 243 tests were performed and 240 sets of data analysed, 3 sets of case notes could not be obtained. 147 (61%) patients presented with wheeze and dyspnoea with a possible diagnosis of asthma, 48/134 (36%) tests were positive confirming the diagnosis and 13 (8.8%) patients were unable to perform the test. 89 (37%) patients presented with cough, 20/86 (23.3%) tests were positive and three patients were unable to perform the test. 68/69 (99%) of the positive mannitol tests were interpreted as confirmation of the diagnosis of asthma. The 155 negative tests were interpreted as false negative in 20 (13%) patients. In 87 (56%) cases additional tests were subsequently performed and an alternative diagnosis was made and in 48 (31%) cases the result was interpreted as true negative. Three of these patients (6%) re-presented and were subsequently diagnosed with asthma.
Conclusion Mannitol challenge tests are useful in confirming the diagnosis of asthma in patients with high pre-test probability of the disease. Physicians need to recognise the risk of false negative mannitol test results and perform additional tests when the diagnosis is uncertain and clinical suspicion remains high.
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