Colonisation or Aspergillosis? That is the Question

Submitted by Aspergillus Administrator on 8 February 2013

Patients who attend hospital who have respiratory disease symptoms are commonly asked for a sputum sample or given a bronchoalveolar lavage (BAL) in order to assess the microbial contents of their airways. Samples taken are cultured and what grows out on any of several media is noted as a possible infecting agent. One difficulty is that some micro-organisms can be found by this method whether or not an infection is present so it is not a diagnostic test when carried out on its own – other tests are usually done at the same time.

If Aspergillus grows out of one of these samples the situation is unclear as this fungus along with many others is inhaled into the airways every day during our normal activities as these fungi release spores that float in the air, indoors and out. There is also a phenomenum known as colonisation which is defined as microbes growing on a surface or tissue (e.g. our airways) without causing infection. We do not really know conclusively if a colonising Aspergillus isolate will go on to cause infection if the patient has a normal immune system.

This new paper looks at 126 patients who came into hospital with chronic obstructive pulmonary disease (COPD) which can involve significant scarring of lung tissue and thus forms a potential area of the lung in which Aspergillus can sometimes grow. These patients were either given a sputum sample test or BAL and Aspergillus was detected. The researchers then looked at how many of those people could subsequently be described as suffering from aspergillosis. 1.6 to 3.2% (depending on which criteria for aspergillosis was used) were found to be suffering from aspergillosis.

Other than this study providing a strong suggestion that there needs to be some harmonization of the different criteria for diagnosing aspergillosis (as aspergillosis tends to occur alongside several other illnesses it is sometimes difficult to assess) it also shows that the detection of Aspergillus in sputum or BAL samples is not by itself a good guide to diagnosis of aspergillosis and is probably indicating colonisation of the airways.