MLPAO Microbiology and Parasitology Diagnostic Methods 2

Question 1

Which staining method is used to identify Pneumocystis jirovecii in respiratory specimens?

Accepted Answer

Gomori methenamine silver (GMS) stain or immunofluorescence

Answer

Gomori methenamine silver (GMS) stain highlights the cyst wall of Pneumocystis jirovecii as dark brown/black against a green background. Direct fluorescent antibody (DFA) testing is more sensitive and specific. PCR is increasingly used as it can detect both cystic and trophic forms. P. jirovecii causes pneumonia in immunocompromised patients.

Question 2

What is the principle of blood culture gram stain and how should results be communicated?

Accepted Answer

Gram stain is performed immediately when the instrument flags positive, and results are communicated urgently to the physician

Answer

When an automated blood culture system flags a bottle as positive, the Gram stain is performed immediately to provide preliminary organism identification. This result must be communicated urgently (STAT) to the physician as it guides empiric antibiotic therapy. The Gram stain morphology (e.g., gram-positive cocci in clusters) is critical for initial treatment decisions.

Question 3

Which serological test is used for the diagnosis of syphilis and what is the screening algorithm used in Canada?

Accepted Answer

Reverse algorithm: treponemal test (EIA/CIA) screen followed by RPR and confirmatory treponemal test

Answer

Many Canadian laboratories use the reverse syphilis screening algorithm: a treponemal-specific immunoassay (EIA or CIA) is used for screening, followed by RPR (a non-treponemal test to assess disease activity) and a second treponemal test (TP-PA or FTA-ABS) for confirmation. This approach has higher sensitivity for detecting latent syphilis.

Question 4

What is the KOH (potassium hydroxide) preparation used for in clinical microbiology?

Accepted Answer

Dissolving keratin and cellular debris to reveal fungal elements in clinical specimens

Answer

KOH preparation dissolves keratin, cellular material, and debris in specimens (skin scrapings, nail clippings, hair) while leaving fungal cell walls intact. This allows visualization of fungal hyphae, pseudohyphae, and yeast cells under microscopy. Adding calcofluor white and viewing under fluorescence microscopy enhances sensitivity.

Question 5

Which test differentiates Enterococcus from Group D Streptococcus?

Accepted Answer

Growth in 6.5% NaCl broth

Answer

Enterococci grow in 6.5% NaCl broth while Group D streptococci (S. bovis group) cannot. Both are bile esculin-positive. This salt tolerance test is important because enterococci have different antibiotic resistance patterns and treatment implications than Group D streptococci.

MLPAO - Medical Laboratory Professionals Association of Ontario Practice Test

MLPAO - Medical Laboratory Professionals Association of Ontario Practice Test

MLPAO Microbiology and Parasitology Diagnostic Methods 2