Laboratory diagnosis of candidiasis (Candida species, most commonly C. albicans):
Direct microscopy — KOH mount or Gram stain of clinical specimen (skin/mucosal scraping, vaginal discharge, sputum, or other body fluid) showing budding yeast cells, with or without pseudohyphae; a Gram stain shows the organism as Gram-positive, oval, budding cells.
Culture — on Sabouraud dextrose agar (SDA), giving cream-coloured, smooth, pasty colonies within 24–48 hours; blood culture for suspected invasive/systemic candidaemia (using specific fungal blood culture bottles/systems for optimal yield).
Germ tube test — the key rapid presumptive identification test for C. albicans: incubation in serum at 37°C for 2–3 hours produces a characteristic germ tube (filamentous outgrowth without constriction at its base); positive in C. albicans (and C. dubliniensis), negative in most other Candida species.
Chlamydospore formation — on cornmeal-Tween 80 agar, C. albicans produces characteristic thick-walled terminal chlamydospores, a confirmatory feature.
CHROMagar Candida — a chromogenic differential medium giving characteristic colony colours for different Candida species (C. albicans — light green), useful for rapid presumptive speciation, especially in mixed infections.
Biochemical/sugar assimilation tests and automated identification systems (e.g., VITEK) for definitive species-level identification of non-albicans Candida species, increasingly important given rising incidence of intrinsically more resistant non-albicans species (e.g., C. glabrata, C. krusei, C. auris).
Antifungal susceptibility testing — important for invasive candidiasis, given variable and rising resistance patterns, particularly for C. auris, a multidrug-resistant emerging pathogen of major infection-control concern.
Histopathology — biopsy (PAS or GMS stain) demonstrating tissue invasion, important for confirming invasive disease versus mere colonization.
Molecular methods (PCR) — increasingly used for rapid species identification and detection directly from blood/sterile-site specimens, particularly valuable given the slow turnaround of conventional blood culture for candidaemia.
