The laboratory diagnosis of CAM remains a real challenge (
52). Diagnostic confirmation can be obtained by mycological (direct examination and fungal culture) and histopathology tests (
53). There are no commercially available biomarkers or serological tests to detect Mucorales (
48). The β-D glucan and galactomannan test does not detect antigen components of the Mucorales (
9,
48). It is advised to collect biopsies of affected tissues in formalin (for a histopathology exam) and normal saline (for mycology examinations) in patients with ROCM or bronchoalveolar lavage (BAL) in patients with pulmonary mucormycosis (
9,
17,
48). However, it can be challenging to take a biopsy in patients with severe thrombocytopenia or unstable systemic condition (
9,
48). In these cases, upon mucormycosis suspicion, collecting serum specimens and other body fluids to detect Mucorales DNA can be attempted (
9). Histopathology and mycological examinations are very important diagnostic tools since they distinguish the presence of the fungus elements (aseptate ribbon-like hyphae with right-angle branching) in the specimen from a culture contaminant and are indispensable to define whether there is blood vessel invasion (
9,
48,
53). Also, these tests can furthermore reveal coinfections with other molds. Routine hematoxylin and eosin (H&E) staining may show only the cell wall with no structures inside. Stains that can help highlight the fungal wall include grocott methenamine-silver (GMS) and periodic acid-Schiff (PAS) stains, though fragmentation and necrosis of the fungal elements may cause these stains, in particular GMS, to be either faintly positive or negative. PAS gives a better visualization of the surrounding tissue compared to GMS. Direct microscopy of fresh material with potassium hydroxide (KOH) wet mounts is an inexpensive and yet invaluable method to rapidly give a presumptive diagnosis of mucormycosis. Immunohistochemistry using monoclonal antibodies against
Rhizopus arrhizus has been proven useful for differentiating aspergillosis from mucormycosis (sensitivity 100% and specificity 100% for mucormycosis), particularly when cultures are negative. Identifying the definite species of the causative agent of mucormycosis to provide antifungal susceptibility tests and help for proper antifungal treatment is marginally recommended (
9). Nonetheless, it should be considered that some genera, such as
Cunninghamella, can be associated with an increased mortality rate in patients and have been shown to be more virulent in experimental models (
9).
Most medically important Mucorales colonies grow at a temperature of 37°C within 24 - 48 hours. There is a caveat for false positive results, especially when histopathology is not available. The positive culture from a non-sterile site must be interpreted along with clinical and radiological data to establish a reliable diagnosis (
49). It should be noted that fungal culture can be falsely negative in up to 50% of mucormycosis cases (
44). Homogenization of tissue specimens may destroy the delicate hyphae or ongoing therapy with antifungal agents effective against Mucorales are rendering cultures negative. In these cases, attention should be paid to molecular techniques as auxiliary diagnostic tools, which can confirm the infection and identify the causal agent (
9,
54). Fresh material is the preferred subject over paraffin embedded tissue because formalin damages DNA (
9). Several molecular methods have been developed for the diagnosis of mucormycosis, including polymerase chain reaction (PCR)-based techniques such as nested PCR, real-time PCR, PCR followed by RFLP, PCR coupled with electrospray ionization mass spectrometry (PCR/ESI-MS), and PCR/high-resolution melt analysis (HRMA) (
55). The 18S ribosomal RNA region and also other targets such as 28S rDNA, the mitochondrial gene rnl, the cytochrome b gene, or the Mucorales-specific CotH gene have been investigated for molecular detection and identification in general. All the diagnostic strategies needed to be stepwise taken for CAM diagnosis are summarized in the
Figure 2.