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Reporting mixed urine culture.

Q Our OB/GYN POL recently switched to a hospital reference lab. When we send a CCMS urine for culture, this lab performs a Gram stain if the dipstick is positive for leukocytes and/or nitrites. Our previous reference lab did not do this. Is this a new standard of care? Also, when reporting culture results, the hospital lab will report a colony count and specific definition of each colony type, (e.g., <10,000 coliform, <10,000 alpha strep, <10,000 second coliform type, <10,000 Lactobacillus). Our previous reference lab would report out as "mixed genitourinary flora," which we would consider as probable contamination. Any thoughts on this?


A Urine culture is the most common test performed by most microbiology laboratories, and most urine cultures are negative. For this reason, screening methods are available that attempt to rapidly separate specimens containing significant counts of bacteria from negative specimens. In general, screening methods perform well with specimens containing at least 100,000 CFU/mL of bacteria but perform poorly when colony counts are lower.

Screening urine specimens by staining with the Gram stain is rapid and economical with regard to reagents, but is labor-intense and requires a trained technologist. Commercially available dipstick tests that detect leukocyte esterase (LE) and nitrite are rapid, inexpensive and simple to perform, but their sensitivity is low in some patient populations. (1) If a dipstick is positive for LE and/or nitrites, this is an indication that the specimen may contain a clinically significant amount of white cells and/or bacteria. Conversely, a urine sample that is negative for LE and nitrites would not likely contain a significant amount of white cells or bacteria.

Performing a Gram stain only on urine samples that are positive for LE or nitrites increases the chance that the specimen will show positive results. This approach can likely provide initial information to the physician on what the causative organism might be, while limiting the labor-intensive Gram stain procedure to those specimens likely to show clinically relevant information. There are several ways to report urine culture results. In many patients, a CCMS urine specimen, which contains less than 10,000 CFU/mL, whether mixed or pure, can be considered nonsignificant, however, there are exceptions to this, and the reader is referred to the reference indicated above for further information. (1) Generally, reporting of less than "10,000 CFU/mL of mixed genitourinary flora" can be interpreted to indicate contamination, again with some exceptions. (1) Reporting a mixture of three or more colony types at less than 10,000 CFU/mL would indicate a similar finding.


1. Thomson RB, Miller JM. Specimen collection, transport, and processing: bacteriology. In: Murray PR, Baron EJ, Jorgensen JH, Pfaller MA, Yolken RH, eds. Manual of Clinical Microbiology, 8th ed. ASM Press, Washington DC. 2003:320-322.

--Susan E. Sharp, PhD, DABMM

Director of Microbiology

Kaiser Permanente

Associate Professor

Pathology Regional Laboratory

Oregon Health Science University

Portland, OR

Edited by Daniel M. Baer, MD

MLO's Tips from the Clinical Experts department provides practical, up-to-date solutions to readers' technical and clinical issues from a panel of experts in various fields. Readers may send questions to Dan Baer by fax, (503) 636-7932; or e-mail,
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Title Annotation:Experts Focus on Lab Concerns
Author:Sharp, Susan E.
Publication:Medical Laboratory Observer
Geographic Code:1U9OR
Date:Feb 1, 2004
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