Important 1st clue to the etiology of infectious diseases
It may provide a presumptive etiologic diagnosis or at least narrow the possibilities.
The Gram-stain influences the use of other diagnostic methods (e.g. specific anaerobic or fungal cultures)
The Gram-stain provides important clues in patients who are partially treated where cultures may be negative.
The interpretation of the Gram-stain can only be as reliable as the technique is adequate. The evaluation of the adequacy of the Gram-stain should take into consideration:
Whether the Gram-stained specimen is properly decolorized
In the case of the sputum Gram-stain, does the smear truly represent lower respiratory tract secretions (e.g. look for inflammatory cells, such as neutrophils or alveolar macrophages)
Is the specimen free from contamination. In the case of the sputum Gram-stain the specimen should have fewer than ten squamous epithelial cells per 100 power field and > 25 neutrophils per 100 power field.
Generally, sterile but infected specimens should contain only 1 or 2 types of bacteria.
Ignore contaminating flora
near epithelial cells.
Artifacts within the Gram-stain
. Gram (+) bacteria may become Gram-variable or Gram (-) after exposure to antibiotics. The reverse is generally not true: Gram (-) bacteria do not become Gram (+) with damage.
Do not over interpret the gram-stain: Gram (+) cocci such as Staphylococci can occur both in clusters as well as short chains. The corollary to over-interpretation is recognizing the limitation of the gram-stain.
KOH Prep of Sputum
A wet mount of sputum incubated with 10 to 20% KOH may be useful in evaluating for the presence of elastin fibers
Elastin fibers suggest the presence of destruction of lung and would be associated with necrotizing pneumonias such as TB, Gram (-) infections and aspiration lung abscess
The presence of Elastin fibers makes non-necrotizing process such as mycoplasma, viral pneumonitis and the typical pneumococcal pneumonia less likely
Acid-fast Stain
Mycobacteria are not visible in the traditional Gram-stain