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June 2005
Richard A. Savage, MD, Editor
Q. Is there a standard set of criteria for “culture
is indicated” for urinalysis specimens?
A. I know of no standard for when to perform a urine culture.
Attending physicians, not laboratorians, usually make the decision based on
their clinical impression and physical findings such as chills, fever, flank
pain, and abnormal laboratory results, especially an abnormal urinalysis. The
lab results often indicate the possibility of urinary tract infection and may
include leukocyturia, hematuria, and the presence of observable bacteriuria
by microscopy.
Meryl H. Haber, MD
Scottsdale, Ariz.
Q. Should cervical Pap tests with hyperkeratosis or parakeratosis (but
no dyskeratosis) be diagnosed as atypical squamous cells-undetermined significance
or as normal? The Bethesda classification states that as long as no dyskeratosis
is present, the Pap test is interpreted as normal. At my institution, the pathologists
state that if there is a large enough quantity of those changes present on the
slide, it should be diagnosed as ASC-US. The rationale for the ASC-US diagnosis
is that dysplasia occasionally can be present underneath hyperkeratosis or parakeratosis.
A. Hyperkeratosis and parakeratosis in isolation represent
a benign change of squamous epithelium. Although you are correct that these
changes may cover an underlying lesion, the finding of hyperkeratosis or parakeratosis
alone is nonspecific and should be included in the category of negative for
intraepithelial lesion or malignancy, or NILM, rather than ASC-US. A typical
parakeratosis (parakeratosis with atypical, enlarged, or irregular nuclei),
in contrast, should be reported as ASC-US or ASC-H (atypical squamous cells,
cannot exclude high-grade squamous intraepithelial lesion) because it may be
associated with a squamous intraepithelial lesion. Such cases should be examined
carefully for squamous intraepithelial lesion.
Theresa M. Voytek, MD
Department of Pathology
Hartford (Conn.) Hospital
Member, CAP Cytopathology Resource Committee
Q. What procedures should be followed for collecting multiple blood
cultures? Are there guidelines pertaining to time intervals and draw sites?
A. Recommendations for collecting blood cultures are based
on clinical and laboratory observations, the results of controlled clinical
trials, and experimental data.1-3 The Clinical
Laboratory and Standards Institute, or CLSI (formerly known as NCCLS), is developing
a draft document containing recommendations for collecting blood cultures, which
should be available within the year. The most critical factor in detecting bacteria
or fungi in blood is the volume of blood cultured. The number of bacteria or
fungi present in blood ranges from less than one to more than 100 microorganisms/mL.
The number tends to be lower for adults and higher for young children. Moreover,
many studies have shown that culturing higher volumes of blood increases the
likelihood of detecting bacteria or fungi.1-4
For routine blood cultures using standard bottles—that is, those without
additives such as resins—the recommended volume for culture is 8 mL to
10 mL per bottle. For bottles with additives, the recommended volume for culture
is 5 mL per bottle. Two or three sets (blood drawn from two independent venipunctures)
of blood cultures should be drawn per septic episode. Drawing more than one
set of blood cultures increases the volume of blood cultured and improves the
clinician’s ability to interpret the results.
The practice of drawing one blood culture set and then waiting an arbitrary
period to draw the next set is not supported by published data.4
Not only does this practice not increase the yield, but it is inefficient for
phlebotomists and inconvenient to patients. All sets should be drawn at the
same time.
Many bacteria that cause bacteremia also contaminate blood cultures.5
For bacteria such as Staphylococcus aureus, coagulase-negative staphylococci,
enterococci, and viridans streptococci, recovery of bacteria from blood is not
sufficient to distinguish between a pathogen and a contaminant. The proportion
of blood culture sets that yield the bacterium is the best criterion for making
this distinction; contaminants typically grow only in one blood culture set,
whereas pathogens typically grow in more than one set.6
The number of positive bottles, on the other hand, is not useful for making
this distinction.7 Because many bacteria that
cause sepsis also are common blood culture contaminants, and because only eight
percent to 10 percent of blood cultures yield pathogens, the clinical interpretation
of blood cultures is confounded when contamination rates are not kept as low
as possible. Contamination rates should be kept at or below two percent to three
percent. This can only be accomplished by good technique; no particular disinfectant
can overcome poor technique.8 Although there
are no published guidelines regarding draw sites, it is best to avoid drawing
blood cultures from sites that are more heavily colonized with bacteria, such
as the groin.
References
1. Dunne WM, Nolte FS, Wilson ML. Cumitech 1B: Blood Cultures III. Hindler
J, coordinating editor. Washington, DC: ASM Press; 1997.
2. Reimer LG, Wilson ML, Weinstein MP. Update on detection of bacteremia and
fungemia. Clin
Microbiol Rev. 1997;10:444–465.
3. Magadia RR, Weinstein MP. Laboratory diagnosis of bacteremia and fungemia.
Infect
Dis Clin North Am. 2001;15:1009–1024.
4. Li J, Plorde J, Carlson L. Effects of volume and periodicity on blood cultures.
J
Clin Microbiol. 1994;32:2829–2831.
5. Weinstein MP. Blood culture contamination: persisting problems and partial
progress. J
Clin Microbiol. 2003;41:2275–2278.
6. Aronson MD, Bor DH. Blood cultures. Ann
Intern Med. 1987;106:246–253.
7. Mirrett S, Weinstein MP, Reimer LG, et al. Relevance of the number of positive
bottles in determining clinical significance of coagulase-negative staphylococci
in blood cultures. J
Clin Microbiol. 2001;39:3279–3281.
8. Wilson ML, Weinstein MP, Mirrett S, et al. Comparison of iodophor and alcohol
pledgets with the Medi-Flex Blood Culture Prep Kit II for preventing contamination
of blood cultures. J
Clin Microbiol. 2000;38:4665–4667.
Michael L. Wilson, MD
Medical Laboratories
Denver (Colo.) Health Medical Center
Member, CAP Microbiology Resource Committee
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