How do we diagnose Invasive Aspergillosis? - Part 2
Following on from the previous article, a combination of radiology, microbiology and clinical diagnostics can be used to aid early diagnosis:
- Detection of Aspergillus DNA in BAL samples has reasonable sensitivity and specificity for IA in patients at risk. If two samples are PCR positive, the specificity of the result rises above 90%.
- The sensitivity of blood galactomannan testing is good in neutropenic patients but poor in non-neutropenic patients.
- The sensitivity of galactomannan testing of respiratory samples (BAL) is often higher than that of culture.
- Antifungal treatment targeting galactomannan synthesis (echinocandins) reduces the sensitivity of the test. When other agents are used for treatment, galactomannan can be used to monitor response to therapy.
- As discussed in Week 1 galactomannan is one of the structural components of many filamentous fungi and not specific to Aspergillus species.
- Fungal glucan may be a useful test when assessing the likelihood of invasive fungal infection such as IA but is not specific to Aspergillus
- It can be used as a rule in test in combination with the galactomannan test
You may recognise some of these tests from the discussion in Case 2. Many fungi share characteristics and therefore the tests used to detect them are similar. This highlights the usefulness of sensitive tests such as β-1-3-D-glucan. For definite diagnosis, a combination of diagnostic approaches is often needed.
Aspergillus is an environmental mould
It is important to consider the role Aspergillus species as an environmental organism. Humans breathe in fungal material on daily basis, and this rarely leads to any disease. Aspergillus species can be isolated from respiratory secretions in the absence of disease. This represents colonisation instead of infection.
Each result has to be considered in clinical context. Distinguishing between incidental and significant results can be a major challenge. Often, specialist advice is required for this.