How do you check for protein in the urine?

How to Check Urine Protein

The preferred method for checking urine protein is using an untimed (spot) urine sample with protein-to-creatinine ratio measurement, which has replaced 24-hour urine collections as the standard approach for detecting and monitoring proteinuria. 1

Initial Screening

• Begin with automated dipstick urinalysis when available; if not available, careful visual dipstick urinalysis is acceptable 1
• A first morning void sample is preferred, but a random specimen is acceptable 1
• Dipstick results are typically reported as negative, trace, 1+, 2+, 3+, or 4+ 2

Confirmation of Positive Results

• If dipstick is positive (≥1+, 30 mg/dL), confirm with spot urine protein/creatinine (PCr) ratio within 3 months 1
• A PCr ratio ≥30 mg/mmol (0.3 mg/mg) is considered abnormal 1
• A negative dipstick test can usually be accepted without further PCr testing at that time 1

Different Testing Approaches Based on Risk

For Persons at Increased Risk for Chronic Kidney Disease:

• Use albumin-specific dipstick or directly measure albumin-creatinine ratio 1
• Even with a negative standard dipstick result, those with diabetes should undergo testing with either an albumin-specific dipstick or albumin-creatinine ratio measurement 1

For Persons Not at Increased Risk:

• Standard dipstick for protein and untimed urine measurements for total protein-creatinine ratio are useful 1

Interpretation of Results

• Normal protein excretion is less than 150 mg/day, with values below 40-100 mg/day considered completely normal 2
• Persistent proteinuria is defined as two or more positive results on quantitative tests over a 3-month period 1
• A dipstick result of 3+ has the best combination of sensitivity (96%) and specificity (87%) in predicting a protein-creatinine ratio ≥1 3

When to Use 24-Hour Urine Collection

• 24-hour urine collection is still considered the gold standard but is rarely necessary 1
• Consider 24-hour collection only in special circumstances, such as:
  • To confirm nephrotic syndrome (which has implications for thromboprophylaxis) 1
  • When there are discrepancies between dipstick results and clinical presentation 1

Pitfalls to Avoid

• Relying solely on dipstick testing for definitive diagnosis - dipstick tests have high specificity but low sensitivity and positive predictive value 4
• Failing to account for urine concentration when interpreting dipstick results - lower specific gravity can affect interpretation of proteinuria severity 3
• Not confirming transient proteinuria, which can occur during menstruation or exercise 2
• Assuming a single negative dipstick excludes all proteinuria - some cases may be missed, especially microalbuminuria 1

Algorithm for Testing

1. Initial screening with urine dipstick
2. If positive (≥1+), proceed to spot urine PCr ratio
3. If PCr ratio ≥30 mg/mmol (0.3 mg/mg), proteinuria is confirmed
4. For persistent proteinuria (two positive tests over 3 months), further evaluation for chronic kidney disease is warranted 1