How do I properly collect and interpret a clean‑catch midstream urine specimen for urinalysis?

How to Assess Urinalysis: Collection and Interpretation

Specimen Collection Technique

For both men and women, midstream clean-catch collection with perineal cleansing is the recommended standard technique for urinalysis and culture. 1

Collection Protocol for Women

• Instruct patients to cleanse the perineal area with soap and water or antiseptic wipes, wiping front to back with a fresh wipe for each stroke—this reduces contamination from 23.9% to 7.8%. 2

• Have patients separate the labia and maintain separation throughout the entire voiding process. 2

• Direct patients to begin urinating into the toilet, then without stopping the stream, collect 10-20 mL of midstream urine in a sterile container. 2

• Avoid first-void specimens for routine culture, as contamination rates are significantly higher despite maintaining high sensitivity (0.97). 2

Collection Protocol for Men

• Instruct patients to cleanse the glans penis with an antiseptic pad prior to voiding to reduce contamination. 1, 2

• For uncircumcised males, retract the foreskin before cleansing and keep retracted during collection. 2

• Direct patients to void the first 100-200 mL into the toilet, then collect 10-20 mL of midstream urine in the sterile container. 2

• Do not collect first-void specimens—midstream collection with cleansing shows large reduction in contamination compared to first-void urine. 1

Special Populations

• For catheterized patients, aspirate urine directly from the catheter sampling port using a sterile syringe and needle—never from the drainage bag, which produces false-negative results in 50-64% of cases. 2

• For febrile infants requiring immediate antimicrobial therapy, use catheterization (sensitivity 95%, specificity 99%) or suprapubic aspiration (contamination rate 1%) rather than bag collection (contamination rate 46-68%). 2

Specimen Handling and Timing

Process urine specimens within 2 hours of collection or refrigerate immediately at 4°C to prevent bacterial overgrowth and compositional changes. 2, 3

• If transport delay exceeds 1 hour, refrigerate the specimen at 4°C or use boric acid preservation systems. 2

• Do not process specimens held at room temperature for more than 4 hours—overgrowth of both clinically significant and contaminating flora occurs. 1

• Refrigeration and boric acid adequately preserve specimens for up to 24 hours prior to processing, though evidence strength is rated as low. 1

• Check specimen temperature within 4 minutes of collection—it should range from 90-100°F (32-38°C) to verify integrity. 2

Urinalysis Interpretation

Physical Examination

• Document appearance and color immediately upon collection to ensure accurate diagnosis. 2

• Cloudy urine often results from precipitated phosphate crystals in alkaline urine, but pyuria can also be the cause. 3

• Strong odor may indicate concentrated specimen rather than urinary tract infection. 3

Chemical Analysis (Dipstick)

Dipstick urinalysis is convenient but prone to false-positive and false-negative results—interpret in clinical context. 3

• Leukocyte esterase has 72-97% sensitivity and 41-86% specificity for UTI. 2

• Nitrite has 19-48% sensitivity but 92-100% specificity—the most specific single indicator. 2, 4

• Specific gravity provides reliable assessment of hydration status and specimen concentration. 3

• Proteinuria requires differentiation: transient proteinuria is typically benign, but persistent proteinuria requires further work-up. 3

Microscopic Examination

• White blood cells >5/high-powered field (HPF) has 90-96% sensitivity and 47-50% specificity for UTI. 2

• Presence of epithelial cells (>few) strongly suggests contamination from skin and mucosal flora, occurring in 30-65% of specimens depending on collection technique. 2, 4

• Microhematuria has a range of causes from benign to life-threatening—glomerular, renal, and urologic causes can often be differentiated by other urinalysis elements. 3

Culture Interpretation

For definitive UTI diagnosis, require both pyuria/bacteriuria on urinalysis AND ≥50,000 CFU/mL of a single uropathogen from properly collected specimen. 2

• The traditional threshold of 10^5 CFU/mL may miss true infections in symptomatic patients with lower bacterial counts. 2

• For clean-catch midstream specimens in men, growth of ≥1,000 CFU/mL of one predominant species best differentiates sterile from infected bladder urine. 1

• Mixed flora (≥2 organisms) most commonly indicates contamination rather than true infection, especially when accompanied by epithelial cells. 2, 5

• Nonhemolytic streptococci (viridans group) alongside epithelial cells strongly suggests contamination from normal skin and mucosal flora. 5

Common Pitfalls to Avoid

• Never use positive cultures from urine bags to diagnose UTI—they must be confirmed by catheterization or suprapubic aspiration due to 65-68% contamination rates. 2

• Do not accept specimens from drainage bags in catheterized patients—this produces unreliable results with high false-negative rates. 2

• Avoid treating asymptomatic patients based on contaminated cultures—this leads to unnecessary antibiotic exposure. 5

• Do not rely solely on automated urinalysis indices in low-prevalence populations—false-positive results are common even with ideal collection technique, except for nitrite which maintains high specificity. 4

• Uncomplicated UTIs diagnosed by positive leukocyte esterase AND nitrite tests can be treated without culture, but either test alone has insufficient specificity. 3