Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1362 page)

   This test should be routinely performed for certain specimen types submitted to the laboratory for bacterial culture (e.g., lower respiratory, wound, tissue, abscess and drainage, sterile fluids, CSF, genital samples).

   Because Gram stain is less sensitive than culture for detection of bacteria, culture should always be performed with Gram stains with a few possible exceptions. Gram stain without culture may provide accurate detection of vaginal and oropharyngeal candidiasis.

   The Gram stain is used for the direct detection and initial presumptive identification of bacteria and yeast in patient specimens. Specimens should be collected and transported according to instructions for specific specimen types. Patient specimens are used to make smears on glass microscope slides. After fixation, slides are sequentially stained with crystal violet followed by iodine solution. The intracellular crystal violet–iodine complexes formed are too large to escape through the thick peptidoglycan cell wall of gram-positive organisms by alcohol decolorization, rendering them dark blue. But the crystal violet–iodine complexes can be rinsed through the thinner, fenestrated cell wall of gram-negative organisms, leaving them colorless. After the rinsing step, gram-negative organisms are counterstained with safranin, resulting in mild to intense pink staining.

   The Gram stain is a differential staining technique. Staining characteristics (e.g., pink or blue) and morphology (e.g., cocci or bacilli) and other characteristics of the primary pathogens are reported. This information may contribute to informed decisions regarding initial empirical therapy.

   The Gram stain may demonstrate host PMNs and other evidence of inflammation. Epithelial cells, derived from mucosal or cutaneous surfaces, predict contamination of the specimen with the patient’s endogenous flora.

   
Turnaround time:
<4 hours.

   Interpretation

   
Expected results:

   Specimens from sterile sites should be negative for microorganisms. Smears from nonsterile sites, such as mucosal surfaces, usually demonstrate organisms of various morphologies typical for the endogenous flora of the site (e.g., respiratory, vaginal, GI).

   PMNs and other signs of an inflammatory reaction are not typical for normal tissue specimens and suggest infection (or other inflammatory condition) at the site of collection.

   
Positive results:

   Microorganisms (usually a single morphotype), in moderate or heavy amounts, with PMNs, and other inflammatory markers are typical of pyogenic infections.