For a fungus, this thing attracts a lot of attention from the college. Question 9 from the second paper of 2014 asked for a substantial amount of detail regarding the diagnosis of aspergillosis.
As with many of these, the Manual offers little help. Instead, one should turn to LITFL's CCC entry on this subject, which is precise and brief. However if one is not into brevity and precision, one can enjoy several hours submerged in the reference swamp listed below. Selected reading could be limited to the following exceptional articles:
Risk factors for aspergillus infection
One of the articles referenced by LITFL contains within it Table 2 (p.207) which lists the risk categories for invasive aspergillosis among ICU patients. In a contorted form, those data are displayed below:
It should be noted (from the same article) that merely being intubated places one at an increased risk of forming Aspergillus colonies in one's respiratory tract. This has the effect of causing positive test results, but no actual invasive disease.
Diagnosis of aspergillosis
Extrapulmonary: immunocompromised host
Difficulty of identifying aspergillosis in the ICU
This answers Question 9 from the second paper of 2014.
Why the diagnosis is not straightforward:
- ICU patients will have nonspecific signs, buried under other signs, and often no history.
- Radiological diagnosis may be obscured by othr pathology, or may be logistically difficult
- Biopsy is frequently impossible
- Immunocompetent individuals are not suspected, yet may still develop the disease
- Clinically insignificant colonisation is common, but will yield the same positive diagnostic results as active invasive disease.
- Diagnostic test accuracy may be confounded by many factors, eg. concurrent β-lactam therapy.
Aspergillus galactomannan: blood
- Galactomannan is a polysaccharide composed of galactose side groups nested on a mannose backbone. It is a ubiquitous organic molecule, and can be found everywhere, including in ice cream (as a texture stabiliser). It is also a fungal cell wall component.
- Sensitivity is 71% and specificity is 89% according to a 2006 meta-analysis.
- Causes of confused results include:
Aspergillus galactomannan: bronchoalveolar lavage
- Sensitivity is 61% and specificity is 98% with a 1.0 cutoff
- Though "specific" for the presence of Aspergillus, this test cannot discriminate among those who have invasive aspergillosis and those who are merely colonised.
Aspergillus PCR on bronchoalveolar lavage specimens
- Old techniques had limited usefulness: poor sensitivity and specificity according to some studies.
- In more recent studies, BAL PCR had higher sensitivity but lower specificity than BAl galactomannan.
Aspergillus hyphae identified
- The gold standard
- Again, as all of these tests, it cannot discriminate among those who have invasive aspergillosis and those who are merely colonised.
In brief, the radiological features are:
- Pulmonary nodule on CXR
- A "halo" of ground-glass opacity surrounds the nodules on CT
- Wedge-like haemorrhagic pulmonary infarcts
- Air crescents following resolution (necrotic lung separating from the rest of the parenchyma)
Open lung biopsy is not to be taken lightly. Furthermore, it is not always helpful. In one 2001 study the authors claim that they could histopathologically confirm invasive fungal infections only in 53.1% by open lung biopsy. Generally speaking these days people rely on laboratory data and radiology. The open biopsy remains at the hopeless end of the diagnostic algorithm flowcharts. Its only benefit is to spare the patient from a course of amphotericin therapy.
Management of Aspergillosis
The Sanford Guide recommends the following:
- Voriconazole (loading dose of 6mg/kg, followed by 4mg/kg)
- Amphotericin B