Understanding Urinalysis

Clues for the Obstetrician-Gynecologist

Sarah Coad; Boris Friedman; Roxana Geoffrion

Disclosures

Expert Rev of Obstet Gynecol. 2012;7(3):269-279. 

In This Article

Urinalysis Handling

Specimen Collection

The 'midstream clean-catch' urine sample is currently the standard collection method.[6] It is thought to decrease sample contamination and false-positive results, especially when the specimen is used to determine the presence of infection.[1,2] With this method of sample collection, the patient is instructed to cleanse the labia and urethral meatus prior to voiding and to commence collection of the sample midway through voiding. Bradbury collected urine samples from 316 women, 158 of which had symptoms suggestive of a urinary tract infection (UTI) and 158 controls. All subjects were randomly assigned to one of two collection methods, involving either preparatory vulvar cleansing or not. There was no difference found in positive culture rates between the two groups (24.7 and 24.6%, respectively, in the symptomatic group and 2.4 and 1.4%, respectively, among controls). There was also no difference in rates of sample contamination, as indicated by the presence of epithelial cells.[7] In a more recent study, Lifshitz and Kramer collected 242 consecutive urine samples from women randomized into three collection technique groups. The first group was told to simply urinate into a clean container, the second group was given the standard clean-catch midstream instructions, and the third group was given the same instructions as the second group with the addition of a vaginal tampon. Contamination rates were calculated for all three groups and no differences were found (29, 32 and 31%, respectively; p = 0.82).[6] This suggests that details of the collection method do not make a difference in the detection of a UTI.

Catheter urine specimens (CSUs) may be appropriate in certain clinical settings, such as in the operating room or when patients are unable to void on command. CSUs are thought to decrease specimen contamination and increase detection of UTI by urinalysis and routine microscopy. In a study of 607 nonpregnant women, culture-proven UTI rates were significantly higher in the CSU group when compared with the midstream urine group (21 vs 15%, respectively; p < 0.001).[8]

In summary, the midstream clean-catch method of urine collection is not recommended for detection of a UTI in nonpregnant women, as it does not decrease contamination rates. CSUs may increase the sensitivity and specificity of urinalysis for the detection of a UTI. More research is needed to verify the use of midstream clean-catch collection and of CSUs in pregnant women and for other urinary tract diseases.

Sample Storage

Following collection, improper specimen handling can affect urinalysis results. The chemical composition of urine is relatively stable over 24 h if bacterial growth is inhibited. This can be accomplished by storing the sample at 4°C with minimal exposure to light.[2,9] Samples should not be kept at room temperature for longer than 2 h.[1] On the other hand, formed elements are particularly unstable. In our hospital laboratory, samples more than 4 h old will not undergo microscopy for this reason. Specimens older than 24 h should not be used for urinalysis.[10]

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