How should a spot urine protein-to-creatinine ratio be collected and measured?

How to Collect and Measure a Spot Urine Protein-to-Creatinine Ratio

Use a first morning void midstream urine sample and measure both urine protein concentration (mg/dL) and urine creatinine concentration (g/dL), then report the result as milligrams of protein per gram of creatinine (mg/g). 1

Specimen Collection Protocol

Timing and Type of Sample

• Collect a first morning void specimen as the preferred sample in both adults and children, as this minimizes diurnal variation in albumin excretion and avoids the confounding effect of orthostatic proteinuria 1
• If a first morning sample cannot be obtained, use a random daytime specimen collected during normal activity hours (after the first void but before bedtime), though this is less accurate 2
• Ensure the patient collects a midstream sample to reduce contamination 1

Pre-Collection Patient Instructions

• Avoid vigorous exercise for 24 hours before collection, as physical activity transiently elevates urinary protein excretion 1
• Avoid collection during menstruation, as menstrual blood contamination falsely elevates protein measurements 1
• Patients should maintain their usual dietary habits unless specifically instructed otherwise 1
• Ensure the patient is not acutely ill with fever, urinary tract infection, or marked hyperglycemia, as these conditions cause transient proteinuria elevation 1

Laboratory Measurement and Reporting

Analytical Methods

• Measure urine protein concentration using quantitative laboratory methods (not dipstick alone) 1
• Measure urine creatinine concentration using either Jaffe or enzymatic assays, though enzymatic methods are preferred in children due to fewer interferences 1
• Calculate the ratio by dividing protein concentration (mg/dL) by creatinine concentration (g/dL) 1

Reporting Format

• Report results as mg protein per g creatinine (mg/g), which is the standard unit in the United States 1
• Alternative units include mg/mg (divide mg/g by 1000) or mg/mmol (multiply mg/mg by 8.84), though mg/g is most widely used 3
• The laboratory should provide a reference range indicating that values <200 mg/g are normal in the general adult population 1

Confirmation Requirements

When to Repeat Testing

• Confirm any elevated result (≥200 mg/g) with a second specimen within 3 months, as day-to-day variability in albumin excretion is substantial 1
• Define persistent proteinuria as 2 out of 3 positive samples collected over a 3-to-6-month period 1
• For patients with diabetes, confirm albumin-to-creatinine ratio values >30 mg/g in 2 of 3 samples before diagnosing microalbuminuria 1

Factors That Affect Interpretation

• Hematuria, menstruation, and urinary tract infection increase both albumin and protein in urine, requiring repeat testing after resolution 1
• Biological sex affects urinary creatinine excretion: females have lower creatinine excretion (causing higher PCR for the same protein excretion), while males have higher creatinine excretion (causing lower PCR) 1
• Body weight extremes (cachexia, muscle atrophy, or extreme obesity) alter creatinine excretion and may make the PCR less reliable, potentially requiring 24-hour collection 1

Pediatric-Specific Considerations

• In children, measure both urine PCR and urine ACR on the same first morning specimen as the preferred initial test 1
• First morning samples are particularly important in children and adolescents to avoid false positives from orthostatic proteinuria 1
• Laboratories measuring creatinine in infants or small children must ensure quality control includes the lowest expected range of values for this age group 1
• Consider consistent use of enzymatic creatinine assays in children given the higher contribution of non-creatinine chromogens when using Jaffe methods, especially in neonates with icteric or hemolyzed samples 1

Common Pitfalls to Avoid

• Do not rely on dipstick urinalysis alone—always confirm positive dipstick results with quantitative PCR measurement 1
• Do not collect specimens during acute illness, marked hyperglycemia, or congestive heart failure, as these conditions cause transient elevations that do not reflect baseline kidney function 1
• Do not interpret a single elevated PCR as definitive—biological variability requires confirmation with repeat testing 1
• Do not routinely order 24-hour urine collections for initial screening or monitoring, as spot PCR is accurate and more practical for most clinical scenarios 1

When 24-Hour Collection May Still Be Needed

• Nephrotic-range proteinuria confirmation (>3.5 g/day) when thromboprophylaxis decisions depend on precise quantification 3
• Patients with extremes of body habitus (cachexia, severe muscle atrophy, or extreme obesity) where creatinine excretion is abnormal 1, 3
• Glomerular disease requiring immunosuppression decisions, where 24-hour collection provides the most precise baseline measurement 3