What urine protein to creatinine (P/C) ratio is diagnostic for pre-eclampsia?

Diagnostic Urine Protein to Creatinine Ratio for Pre-eclampsia

A urine protein to creatinine (P/C) ratio of ≥30 mg/mmol (0.3 mg/mg) is diagnostic for significant proteinuria in pre-eclampsia according to current guidelines. 1

Understanding Proteinuria Assessment in Pre-eclampsia

Proteinuria assessment is a critical component in the evaluation of pregnant women with suspected pre-eclampsia. While proteinuria is no longer required for the diagnosis of pre-eclampsia, its presence helps confirm the diagnosis and may correlate with disease severity.

Diagnostic Thresholds:

• Spot urine P/C ratio threshold: ≥30 mg/mmol (0.3 mg/mg) 1
• 24-hour urine protein threshold: ≥300 mg/24 hours 1
• Dipstick threshold: ≥1+ (approximately 30 mg/dL) 1

Clinical Application and Interpretation

Preferred Testing Method:

• Initial screening: Automated dipstick urinalysis (when available) 1
• Confirmation: Spot urine P/C ratio if dipstick is positive (≥1+) 1
• Gold standard: 24-hour urine protein collection (≥300 mg/24h), though this is being increasingly replaced by spot P/C ratio due to convenience and rapid results 1

Important Clinical Considerations:

1. Reliability of negative results: A negative dipstick test can generally be accepted without further testing at that time 1

2. Limitations of P/C ratio:

   • Small number of proteinuric cases may be missed by negative dipstick test 1
   • P/C ratio <30 mg/mmol occasionally gives false-negative results, but in such cases total protein excretion is usually <400 mg/day 1
   • P/C measurements are unreliable in the immediate postpartum period regardless of delivery mode 2

3. Correlation with disease severity:

   • Massive proteinuria (>5 g/24h) is associated with more severe neonatal outcomes 1
   • Spot P/C ratio >900 mg/mmol (or >500 mg/mmol if age >35 years) is associated with worse maternal outcomes 1

Special Circumstances

Gestational Proteinuria:

• New onset of proteinuria in pregnancy without other features of pre-eclampsia
• Requires monitoring as it may be the first sign of developing pre-eclampsia 1

Nephrotic Syndrome:

• 24-hour urine collection is still indicated to confirm nephrotic syndrome, which has implications for thromboprophylaxis 1

Postpartum Assessment:

• The standard 0.3 mg/mg threshold is unreliable in the immediate postpartum period due to physiologic proteinuria after delivery 2
• Both vaginal and cesarean deliveries show significant increases in proteinuria postpartum 2

Diagnostic Algorithm

1. Initial assessment: Perform automated dipstick urinalysis

   • If negative → No further testing needed at that time
   • If positive (≥1+) → Proceed to spot P/C ratio testing

2. Spot P/C ratio testing:

   • If ≥30 mg/mmol (0.3 mg/mg) → Significant proteinuria confirmed
   • If <30 mg/mmol but clinical suspicion remains high → Consider 24-hour collection

3. 24-hour urine collection (when needed):

   • Indicated for suspected nephrotic syndrome
   • Gold standard but being replaced by spot P/C ratio in most clinical scenarios
   • Significant if ≥300 mg/24 hours

By following this evidence-based approach to proteinuria assessment, clinicians can accurately diagnose pre-eclampsia and implement appropriate management to reduce maternal and fetal morbidity and mortality.