The urine protein-to-creatinine ratio has replaced the 24-hour collection for good reasons: collections are incomplete far more often than anyone admits, and an incomplete collection understates protein without announcing it. But a spot ratio is only as good as which spot it came from, and the request rarely says.
Use a first-morning sample, taken after the patient has been recumbent overnight and before they have been up and about. It removes orthostatic proteinuria, which can double a reading in a young person and lead to a biopsy that was never needed, and it removes the effect of exercise, which raises protein transiently. It also gives a more stable creatinine denominator than a random afternoon sample after fluid intake.
When a result surprises you, ask three questions before repeating it. Was it a first-morning sample? Was the patient febrile, menstruating, or recently exercising, each of which raises protein? And is the creatinine denominator plausible for the person, since a very low urine creatinine in a frail patient inflates the ratio in the same way a low serum creatinine deflates their eGFR. Where the decision is important, two first-morning samples on separate days settle far more than one 24-hour collection.
- Write first-morning sample on the request, not just urine PCR
- Repeat on a second morning before acting on a single abnormal ratio
- Ask about fever, menstruation and exercise before believing a raised result
- Check the urine creatinine itself is plausible; a low denominator inflates the ratio
- Use albumin-to-creatinine ratio where the question is diabetic or early glomerular disease, and protein-to-creatinine where tubular or overflow proteinuria is possible
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