27% of patients with "physician-diagnosed asthma" had a negative methacholine test - misdiagnosis?

The frequency of adults reporting a history of asthma is rising. However, it is unclear whether this increased prevalence is due to a rising trend ("true asthma") or if it reflects an increase in asthma awareness (but no "real asthma").

Data from methacholine challenge, spirometry, and physician assessment were analysed from 304 adults who reported physician-diagnosed asthma.

27% of the patients had a negative methacholine test.

A negative test was associated with:

- an adult-onset of symptoms

- normal forced expiratory volume in 1 s (FEV1)

- no history of exacerbation requiring oral steroids

60% of those with a negative test reported weekly asthma-like symptoms (cough, dyspnea, chest tightness, or wheeze).

39% reported emergency department visits for asthma-like symptoms.

A sizeable percentage (27%) of subjects who report physician-diagnosed asthma have a negative methacholine challenge test. These subjects often have a diagnosis of asthma as an adult and normal or near normal spirometry.

Caution should be exercised in diagnosis of adults presenting with asthma-like symptoms, because they may not have asthma. Bronchoprovocation testing is warranted in this patient group, especially if their spirometry is normal.

If methacholine challenge is negative but FeNO is higher than 30 ppb, perform adenosine challenge to rule out asthma (Ann of Allergy and Imm, 2012).

Differential diagnosis of cough, a simple mnemonic is GREAT BAD CAT TOM. Click here to enlarge the image: (GERD (reflux), Laryngopharyngeal Reflux (LPR), Rhinitis (both allergic and non-allergic) with post-nasal drip (upper airway cough syndrome), Embolism, e.g. PE in adults, Asthma, TB (tuberculosis), Bronchitis, pneumonia, pertussis, Aspiration, e.g foreign body in children, Drugs, e.g. ACE inhibitor, CF in children, Cardiogenic, e.g. mitral stenosis in adults, Achalasia in adults, Thyroid enlargement, e.g. goiter, "Thoughts" (psychogenic), Other causes, Malignancy, e.g. lung cancer in adults).

References:

Negative methacholine challenge tests in subjects who report physician-diagnosed asthma. McGrath KW, Fahy JV. Clin Exp Allergy. 2010 Nov 24. doi: 10.1111/j.1365-2222.2010.03627.x.

Image source: Spirometry, from Wikipedia, the free encyclopedia, GNU Free Documentation License.

Twitter comments:

@frozenwarning: Negative methacholine challenge on one day doesn't preclude asthma. Also, variation of 2 doubling doses. Not impressed with study.