Spot Urine Protein-to-Creatinine Ratio Parameters for Proteinuria
A spot urine protein-to-creatinine ratio (PCR) ≥200 mg/g (0.2 mg/mg) is abnormal and indicates significant proteinuria in the general population, while in pregnancy a threshold of ≥300 mg/g (0.3 mg/mg) should be used. 1
Diagnostic Thresholds by Clinical Context
General Adult Population
- Normal range: PCR <200 mg/g (<0.2 mg/mg) 1, 2
- Abnormal proteinuria: PCR ≥200 mg/g (≥0.2 mg/mg) 1, 3
- Moderate proteinuria: PCR 1,000-3,000 mg/g (1.0-3.0 mg/mg), warranting nephrology evaluation 4
- Nephrotic-range proteinuria: PCR >3,500 mg/g (>3.5 mg/mg), requiring immediate nephrology referral 4, 2
The National Kidney Foundation established these cutoffs based on correlation with 24-hour urine protein excretion, where the PCR in mg/mg approximates grams of protein excreted per day 1, 2. Research demonstrates excellent correlation (r=0.832-0.85) between spot PCR and 24-hour collections 3, 5.
Pregnancy-Specific Thresholds
- Normal range: PCR <300 mg/g (<0.3 mg/mg or <30 mg/mmol) 1
- Abnormal proteinuria: PCR ≥300 mg/g (≥0.3 mg/mg or ≥30 mg/mmol) 1, 6
- Massive proteinuria: >5,000 mg/24h, associated with worse maternal and neonatal outcomes 1
The International Society for the Study of Hypertension in Pregnancy uses a higher threshold (30 mg/mmol) specifically for preeclampsia risk assessment, as pregnancy physiologically increases protein excretion 1, 3.
Collection and Testing Methodology
Optimal Specimen Collection
- First morning void is preferred for children and adolescents to avoid orthostatic proteinuria 1
- Random daytime specimens are acceptable for adults, though samples collected after the first void and before bedtime show best correlation 2
- Avoid vigorous exercise for 24 hours before collection, as this causes transient elevation 1, 4
- Avoid collection during menses due to contamination risk 4, 6
Laboratory Reporting Standards
- Report as milligrams of protein per gram of creatinine (mg/g) 1
- Alternative units: mg/mg (divide mg/g by 1000) or mg/mmol (multiply mg/mg by 8.84) 1, 3
- Refrigerate samples for same-day or next-day assay; one freeze is acceptable if necessary 1
Confirmation Requirements
When to Confirm Proteinuria
- Repeat testing is mandatory if initial PCR is elevated, as transient proteinuria is common 1, 4
- Persistent proteinuria is defined as 2 of 3 positive samples over 3 months in non-pregnant patients 1, 6
- In diabetes mellitus, confirm values >30 mg/g albumin-to-creatinine ratio in 2 of 3 samples 1
When 24-Hour Collection Remains Indicated
Despite PCR accuracy, 24-hour urine collection is still needed to:
- Confirm nephrotic syndrome (>3.5 g/day) for thromboprophylaxis decisions 1
- Resolve discrepancies between PCR and clinical presentation 6
- Evaluate patients with extremes of body habitus (cachexia, muscle atrophy, extreme obesity) where creatinine excretion is abnormal 1
Common Pitfalls to Avoid
- Do not diagnose proteinuria on dipstick alone: A positive dipstick (≥1+) requires quantitative PCR confirmation 1, 6
- Do not ignore false negatives: PCR <30 mg/mmol occasionally misses proteinuria, though usually <400 mg/day 1
- Do not use albumin-to-creatinine ratio interchangeably: While preferred for diabetic nephropathy screening, total protein PCR is required for general proteinuria assessment 1
- Do not overlook benign causes: Exclude UTI, fever, dehydration, and recent exercise before pursuing extensive workup 4, 7
Risk Stratification for Referral
Immediate nephrology referral indicated when: