• Guest speaker Professor Malcolm Richardson

Direct microscopy

Direct microscopy for fungal elements is performed on blood, CSF, tissue and respiratory samples to diagnose IFI and swabs (vulvovaginal, oral), skin scrapings and nail clippings to diagnose of mucocutaneous infections

Potassium hydroxide - breaks down tissue

Common stains:

• Gram stain - performed on blood cultures → detection of fungaemia (Candida spp., Fusarium)
• India ink - highlights capsule of Cryptococcus spp. (although any dark ink can be used if don’t have India ink)
• Fluorescent stains - eg. calcofluor white - enhances sensitivity of direct microscopy
• Immunofluorescent stains - allow specific detection of particular fungal pathogens - commonly used to detect Pneumocystis jirovecii

Benefits:

Direct microscopic examination of clinical samples for fungal elements offers important diagnostic benefits over culture alone:

• Rapid results - often within hours of specimen arrival in the lab
• Provisional identification of fungi with distinctive features → affects antifungal choice
• Aids differentiation of infection from colonisation / culture contamination
• May allow detection of fungi that fail to grow on subsequent culture

Pitfalls

• Lacks sensitivity - false negative results may occur and a negative result does not rule out fungal infection