Q & A

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