What protein-to-creatinine ratio is indicative of preeclampsia?

Protein-to-Creatinine Ratio for Diagnosing Preeclampsia

A spot urine protein-to-creatinine (P/C) ratio ≥30 mg/mmol (0.3 mg/mg) is indicative of significant proteinuria in preeclampsia according to current guidelines. 1

Diagnostic Thresholds for Proteinuria in Preeclampsia

The International Society for the Study of Hypertension in Pregnancy (ISSHP) provides clear guidance on proteinuria assessment in preeclampsia:

• Initial screening: Automated dipstick urinalysis (when available)

  • If positive (≥1+, 30 mg/dL): Proceed to spot urine P/C ratio
  • If negative: Further P/C testing usually not required at that time

• Diagnostic threshold: P/C ratio ≥30 mg/mmol (0.3 mg/mg) is considered abnormal 1

Correlation with 24-Hour Urine Collection

While 24-hour urine protein ≥300 mg has traditionally been the gold standard for diagnosing significant proteinuria in pregnancy, spot P/C ratio offers several advantages:

• Eliminates difficulties in collecting 24-hour urine samples
• Speeds up clinical decision-making
• Shows strong correlation with 24-hour protein excretion (correlation coefficient r=0.758) 2

Different Proteinuria Thresholds and Clinical Implications

Research has established various P/C ratio thresholds that correlate with different levels of 24-hour proteinuria:

• 0.3 mg/mg (30 mg/mmol): Correlates with 300 mg/24h proteinuria
• 0.9 mg/mg: Strongly predicts proteinuria >1g/24h (sensitivity 91%, specificity 95.4%) 2
• 1.16 mg/mg: Correlates with 2000 mg/24h proteinuria
• 2.28 mg/mg: Correlates with 4000 mg/24h proteinuria 2

Clinical Application and Limitations

When interpreting P/C ratios, consider these important caveats:

• False negatives: A small number of proteinuric cases may be missed by P/C ratio <30 mg/mmol, but in such cases, total protein excretion is usually <400 mg/day 1
• Massive proteinuria: Levels >5 g/24h are associated with more severe neonatal outcomes and earlier delivery 1
• Diagnostic accuracy: P/C ratio predicts proteinuria >2000 mg/24h better than it predicts levels between 300-2000 mg/24h 3

Recommendations for Clinical Practice

1. Initial assessment: Use automated dipstick urinalysis when available
2. Follow-up testing: If dipstick is positive (≥1+), obtain spot P/C ratio
3. Diagnostic threshold: Use P/C ratio ≥0.3 mg/mg as indicative of significant proteinuria
4. Severe cases: Consider higher thresholds (≥0.9 mg/mg) as strongly predictive of proteinuria >1g/24h
5. Monitoring: If proteinuria is diagnosed but subsequent dipstick tests become negative, perform further quantification tests 1

Important Considerations

• Proteinuria is not required for a diagnosis of preeclampsia according to current guidelines 1
• 24-hour urine collection is still indicated to confirm nephrotic syndrome, which has implications for thromboprophylaxis 1
• Avoid measuring P/C ratio after vigorous exercise (wait 24 hours) or during urinary tract infections 4
• The degree of proteinuria provides additional risk stratification - massive proteinuria (>5 g/24h) is associated with worse maternal and neonatal outcomes 1

By using the P/C ratio threshold of ≥0.3 mg/mg (30 mg/mmol), clinicians can quickly identify significant proteinuria in preeclampsia and implement appropriate management without the delay associated with 24-hour urine collections.