WHY SHOULD PHYSICIANS
EXAMINE GRAM-STAINED SMEARS?
Statistically about one third of all patients admitted to general
hospitals have, or develop, infections, a large proportion of
which are caused by bacteria and fungi. In most cases, the organisms
cannot be identified by clinical presentation alone. The investment
of time spent in preparing and examining a Gram-stained smear
when a specimen is first obtained often yields valuable dividends
that cannot be obtained from culture of the specimen alone. Gram
stain of collected specimens may help:
-
Determine the adequacy of the specimen for culture
The Gram-stained smear is useful in judging the adequacy of the
specimen obtained. In sputum and urine specimens, for example,
a poorly collected or contaminated specimen can be recognized
by the presence of many epithelial cells in the smear. Instead
of spending laboratory effort and the patient's money on a culture
that may yield worthless or misleading information, a better specimen
should be obtained.
-
Make a presumptive etiologic diagnosis and early clinical decisions
Immediate examination of a Gram-stained smear of material from
the infection site can often provide important data on which to
base early clinical decisions, prior to the availability of culture
results. In certain rapidly progressive infections such as gas
gangrene or acute meningitis, the Gram-stained smear may allow
a presumptive etiologic diagnosis to be made within minutes, whereas
culture results usually are not available for one to two days.
Information gleaned from the Gram-stained preparation rarely
permits definitive identification of organisms, but usually narrows
the possibilities in diseases such as gas gangrene, pneumonia
or meningitis, that have a variety of causative agents. Early
diagnostic information obtained from Gram-stained smears often
allows the physician to prescribe narrow-spectrum antimicrobial
therapy, thereby reducing the risk of toxicity, superinfection,
and the expense of broad-spectrum "poly-pharmacy."
-
Suggest a need for non-routine laboratory procedure
The Gram-stained smear may indicate a need for laboratory procedures
not routinely employed, such as anaerobic and fungal cultures
or special staining techniques, without which the organism might
be missed.
-
Help make accurate interpretation of culture results
The Gram-stained smear may provide clues that are important in
interpreting culture results. In patients who have already received
antibiotics, the direct smear may show organisms that will not
grow in culture. Moreover, in certain infections, such as Vincent's
angina (associated with fusobacteria and spirochetes), the organisms
are not identifiable by the culture techniques employed in most
diagnostic microbiology laboratories, and the Gram-stained smear
together with the clinical findings form the basis for diagnosis.
-
Provide a better insight into the nature of the current infection
In most cases, the Gram-stained smear may reflect what is happening
in the patient better than a culture. In mixed infections, due
to several types of aerobic and anaerobic bacteria, the smear
may indicate the relative abundance of different bacteria, whereas
in culture, the bacteria may grow at different rates, giving a
false quantitative picture. Estimates made regarding the total
quantity of organisms present can sometimes be made from the Gram-stained
smear.
For all these reasons, in the diagnosis of patients with acute
infections, the decision to send specimens of sputum, urine,
cerebrospinal fluid, or material from wounds or abscesses
for culture should automatically trigger a response to first examine
a Gram-stained smear. At times when Gram-staining cannot be done
immediately, as in the operating room, a smear can be made on
a clean glass slide and saved for later staining.
There are some specimens that are not suitable for routine
Gram staining.
Such examples are routine throat and stool specimens
in which the pathogen usually cannot be distinguished from the
plethora of normal flora. Blood specimens are rarely Gram-stained
(although in acutely septic patients a Gram-stained smear of the
buffy coat may be useful).
Certain bacteria stains only feebly or not at all with the Gram
staining technique requring special staining techniques. Such
examples are acid-fast staining for mycobacteria and nocardia,
and immunofluorescent stain for Legionella.
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