Laboratory methods for the diagnosis of invasive mycoses

Invasive mycoses are serious complications in oncohaematologic and surgical patients. Most of them are caused by Candida but other emergent fungi are changing the disease spectrum. Absence of specific symptoms complicates diagnosis. Recovery of fungi from blood and deep specimens remains an insensitive diagnosis, leading to delayed treatment and high mortality. Management of invasive mycoses presents a therapeutic and diagnostic challenge because prompt initiation of targeted antifungal treatment is lifesaving.

An ideal test for diagnosing an invasive fungal infection should have good performance characteristics, give a high level of certainty with respect to the presence or absence of disease, and should become positive at an early stage of infection. However, culture lacks sensitivity, histology requires invasive procedures, imaging lacks specificity, and fungal biomarkers detection requires higher standardization.

There is not available a single test with all these features, and combining information from different laboratory tests and clinical procedures is required. Biomarkers, such as 1,3-β-D-glucan (BG), galactomannan (GM), glucuronoxylomannan (GXM) and mannan, anti-Candida antibodies, or fungal DNA can be useful diagnostic tools.

BG is a panfungal marker that can be present early in the blood and other biological fluids from patients suffering from invasive fungal infections. GM may be detected in the course of invasive aspergillosis and be positive prior to the clinical and radiological suspicions for infection. GXM is essential in the diagnosis of cryptococcal meningitis. Mannan and anti-Candida antibody detections are helpful for the diagnosis of invasive candidiasis. DNA detection by PCR is a promising diagnostic tool under validation before recommendation for clinical use.

Non-culture methods based in biomarkers detection, allow rapid and more-sensitive diagnosis, compared with traditional microbiological diagnosis. Their usefulness increases when samples from patients are tested regularly. BG and GM detection are useful tests for the diagnosis of invasive aspergillosis in high-risk patients with haematological malignancies. In addition, combination of GM and PCR-based testing could also help identify cases of invasive aspergillosis with poor prognosis. One possible diagnostic approach would be to screen high-risk patients for the presence of biomarkers during the periods of highest risk. This selection is very important, because the incidence of invasive mycoses, especially when it is very low, greatly influences the positive predictive and negative predictive values of diagnostic methods. For instance, when the incidence of invasive aspergillosis is 0.5%, as in kidney transplant recipients, only one out of 10 patients with a positive GM and one out of 70 with a positive PCR will actually suffer for invasive aspergillosis.

Although some questions need to be solved, such as kinetics and release of biomarkers, assay performances in different groups of patients, or the effects of antifungal treatment, the broader use of these assays could help to understand better the pathogenesis of invasive mycoses and improve their diagnosis, treatment and prognosis. Moreover, currently most tests lack standardization, and a future consensus will be necessary for facilitating large, prospective, clinical trials focused on the integration of these techniques into clinical decision-making.

In conclusion, the implementation of non-culture based methods, alone or in combination with other microbiological tests, into the clinical setting could improve therapeutic approaches and outcome, by treatment monitoring, and personalizing the therapy.

Additional information available at Enfermedades Infecciosas y Microbiología Clínica

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