Urinalysis and bacterial culture

Extract

• A routine urinalysis consists of chemical screening with multiple test strips, also called dipsticks, and particle counting.
  • In chemical screening, there are several reagent pads on the strip, for detecting leucocytes (esterase activity), erythrocytes (haemoglobin peroxidase activity), bacteria (nitrite test to detect nitrate reducing pathogens) and protein (albumin) in urine. The relative density of urine is important for assessing the concentration of the sample. In addition, testing for glucose, ketones and pH may sometimes be useful.
    • If a urinary tract infection (UTI) is suspected, the sensitivity of the leucocyte and nitrite tests combined is only 40–50% compared with 10³ colony-forming units (CFU)/ml in bacterial culture but approximately 80–90% compared with 10⁵ CFU/ml. The sensitivity of a strip test in detecting pyuria (leucocyturia) in an infant with systemic signs or symptoms (bacteraemia) is 95%, if 5 x 10⁴ CFU/ml is considered a significant result in urine culture, as it is in the USA.
    • The analytical sensitivity of the leucocyte test is about 20 x 10⁶ cells/l (corresponding to 2–3 cells/high-power field, HPF, in traditional sediment microscopy).
    • The analytical sensitivity of an erythrocyte test done with an automatic counter is about 10 x 10⁶ cells/l (1–2 cells/HPF; in traditional microscopy, some erythrocytes disappear during centrifugation of the sediment).
    • The sensitivity of the protein test is about 200 mg/l or 100 mg/l of albumin. A urine albumin-to-creatinine ratio (ACR) sensitive for at least 10 mg/l albumin should be used for screening for incipient albuminuria.
    • The relative density of urine is determined chemically on a dipstick. However, the dipstick reaction is insensitive for dilute samples (relative density of urine 1.000–1.005). Automated strip reading devices used in laboratories therefore measure the relative density by refractometry. The relative densities of isotonic and concentrated urine are 1.010–1.015 and > 1.015, respectively.
• The basic particle count includes urine leucocytes, erythrocytes, various epithelial cells, casts and bacteria. The test is used for investigating UTI, haematuria and renal diseases. Both flow cytometric and automatic imaging devices provide more accurate results than dipstick tests. In smaller laboratories, urine particle counting is done visually by microscopy. A flow cytometer measures the conductivity of the sample, providing its approximate osmolality.
  • Reference limits: urinary leucocyte count below 10 × 10⁶/l (diagnostic grey zone 10–30 × 10⁶/l), urinary erythrocyte count below 10–20 × 10⁶/l (diagnostic grey zone 20–50 × 10⁶/l), bacteria in urine negative (the technical limits for a positive result vary by device). Less than 5 × 10⁶/l epithelial cells or casts in urine (specific limits vary by device).
  • In Finnish regional patient materials, the sensitivity of particle counting as compared to significant bacterial culture may be as high as 95%.
  • The concentration of urine, i.e. relative density or osmolality based on conductivity, is considered when interpreting the results. The osmolality of concentrated urine is > 400 mOsm/kgH₂O.
• The cause of the UTI and its antimicrobial sensitivity are confirmed by bacterial culture of urine. The culture should be requested from the first specimen obtained before prescribing antimicrobials. Random cystitis in otherwise healthy adult female patients need not be confirmed by urine culture; clinical confirmation by a questionnaire is sufficient (use locally available version of the Acute Cystitis Symptom Score https://www.acss.world1). After receiving a positive culture result, a microbiological laboratory will proceed to identify significant bacterial species and to define antimicrobial sensitivity by further techniques.
  • Results of bacterial culture of urine have been received more rapidly over recent years for several reasons.
    • Based on urine particle counting (leucocytes and bacteria), laboratories may release probably negative results in bacterial culture without actual culture.
    • The primary culture plate for uropathogens used today is a chromogenic plate, where E. coli, for example, can be detected directly in the screening culture based on the colour of the colonies.
    • In large laboratories, most uropathogens can be detected by mass spectrometry for bacteria, i.e. MALDI-TOF MS (matrix-assisted laser desorption ionisation time-of-flight mass spectrometry).
• A urine sample for cytological studies should be ordered when a malignant neoplasm is suspected; see Renal cancer Kidney cancer1, Bladder cancer Bladder cancer2. The sample should be taken from fresh urine (second sample in the morning, i.e. urine that has been retained in the bladder for 2–3 hours).

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Bacteriuria, Catheterization, Child, Clinical chemistry, Clinical microbiology, Cystitis, Cytology, Enterococcus, General practice, Hematuria, Internal medicine, Laboratory Techniques and Procedures, Microbiology, N02.9, N10, N30, N39, N39.1, N39.2, Nephrology, Pyuria, R31, R80, R82*, Specimen Handling, Urinalysis, Urinary Catheterization, Urinary Tract Infections, Urine, asymptomatic bacteriuria, bacterial culture, bag urine, bladder punction, bladder puncture, catheter sample, clean-voided urine specimen, collection of a specimen, dipstick test, mid-urine sample, nitrite test, staphylococcus saprophyticus, urine culture, urine sample