Does a urine sample for culture need to be refrigerated at 2-8°C (36-46°F) over the weekend if it can't be sent to the laboratory until Monday, particularly in a patient suspected of having a urinary tract infection (UTI)?

Urine Culture Refrigeration Requirements

Yes, a urine sample for culture must be refrigerated at 2-8°C (4-10°C) if it cannot be sent to the laboratory until Monday, as refrigeration maintains stable bacterial colony counts for up to 24 hours and prevents false-positive results from bacterial overgrowth. 1, 2

Immediate Action Required

• Refrigerate the specimen immediately if processing will be delayed beyond 1 hour from collection 3, 2
• Maintain temperature at 4-10°C (approximately 2-8°C as you specified) throughout the storage period 1
• Label the specimen with collection time to document the delay 3

Evidence Supporting Refrigeration

Refrigerated specimens show no significant change in colony counts when cultured at 0,5, and 24 hours, maintaining diagnostic accuracy for urinary tract infections 1, 2. In contrast, specimens kept at room temperature for more than 2 hours produce colony count changes of ≥1 log₁₀ in approximately 32% of specimens, directly causing misdiagnosis 2.

Specific Performance Data:

• High bacterial counts (≥10⁵ CFU/mL): Refrigeration for 24 hours maintains 82.6-87.3% agreement with immediate processing 2
• Moderate bacterial counts (10⁴-10⁵ CFU/mL): Refrigeration for 24 hours shows 45.8-55.2% agreement, which is still superior to room temperature storage 2
• Room temperature storage beyond 2 hours: Results in false-positive results in 50% of samples and changes clinical interpretation in 8-16% of specimens 2, 4

Critical Time Thresholds

Without refrigeration:

• Maximum 1 hour at room temperature before bacterial overgrowth begins 1, 3
• Beyond 2 hours at room temperature, diagnostic errors occur in up to 32% of positive cultures 2

With refrigeration:

• Up to 24 hours is acceptable for culture specimens with high bacterial loads 1, 2
• Optimal window is 4 hours for urinalysis components (microscopy, dipstick), though culture remains stable longer 1, 3

Common Pitfalls to Avoid

• Never keep specimens at room temperature over the weekend - this guarantees bacterial overgrowth and false-positive results 2, 4
• Do not freeze the specimen - freezing damages cellular elements and is inappropriate for culture 3
• Avoid transport tubes with preservatives (like boric acid) as a substitute for refrigeration when refrigeration is available, as these may show decreased colony counts compared to properly refrigerated specimens 1

Special Considerations for Weekend Storage

For specimens that must wait until Monday:

• Refrigeration at 4°C prevents bacterial overgrowth more effectively than any alternative 2
• Document the storage duration in the laboratory requisition so results can be interpreted appropriately 3
• Be aware that cold storage may cause uromodulin polymer formation, which can trap some cellular elements, but this does not significantly affect culture results for UTI diagnosis 1, 3

When Refrigeration Fails or Is Unavailable

If the specimen cannot be refrigerated and more than 1 hour has elapsed:

• Recollect the specimen rather than sending an unreliable sample 3, 2
• Consider using a commercial preservative tube system only if refrigeration is truly impossible, though this is inferior to refrigeration 1