What is the difference between urinary protein creatinine (PCR) ratio and urinary albumin creatinine (ACR) ratio?

Difference Between Urinary Protein-Creatinine Ratio (PCR) and Urinary Albumin-Creatinine Ratio (ACR)

The key difference is that ACR measures only albumin (a specific protein), while PCR measures total urinary protein (which includes albumin plus all other proteins), and ACR is the preferred test for most clinical situations because it can be standardized, is more accurate in the lower range, and is more sensitive for detecting early kidney damage. 1

What Each Test Measures

Albumin-Creatinine Ratio (ACR)

• Measures only albumin, the predominant protein filtered in most kidney diseases, indexed to urine creatinine to account for urinary concentration 1
• Can be standardized using immunochemical techniques, making it more reliable across different laboratories 1
• More accurate in the lower range of protein excretion, which is critical for detecting early kidney damage 1

Protein-Creatinine Ratio (PCR)

• Measures total urinary protein, including albumin plus other proteins such as tubular proteins, immunoglobulins, and other plasma proteins 1, 2
• Cannot be standardized as easily as ACR due to methodological variability 1
• May detect non-albumin proteinuria that could indicate tubular defects, tubulointerstitial disease, or paraprotein disorders 1

Clinical Interpretation Thresholds

ACR Categories (KDIGO 2020)

• A1 (Normal to Mildly Increased): <30 mg/g 1
• A2 (Moderately Increased): 30-299 mg/g 1
• A3 (Severely Increased): ≥300 mg/g 1

PCR Equivalents

• Normal: <150 mg/g (equivalent to ACR <30 mg/g) 1
• Moderately Increased: 150-499 mg/g (equivalent to ACR 30-299 mg/g) 1
• Severely Increased: ≥500 mg/g (equivalent to ACR ≥300 mg/g) 1

When to Use Each Test

ACR is Preferred For:

• Diabetic kidney disease screening and monitoring, where it is more clinically meaningful than PCR 2
• Early CKD detection, as it is more sensitive for detecting early kidney damage 3
• Initial screening in patients at risk for CKD, using either albumin-specific dipstick or direct ACR measurement 3
• Annual monitoring in established CKD, as recommended by KDIGO guidelines 1

PCR May Be Used When:

• ACR is not available, though predicted ACR from PCR can help with CKD screening and staging 4
• Non-albumin proteinuria is suspected, such as in tubular disorders or paraprotein-related conditions 1
• Some guidelines suggest PCR for non-diabetics, though this is not universally accepted 5

Relationship Between ACR and PCR

• PCR values are typically higher than ACR because total protein includes albumin plus other proteins 6
• The association between PCR and ACR is inconsistent for PCR values <50 mg/g, making conversion equations unreliable at lower levels 4
• For PCR values ≥50 mg/g, conversion equations show moderate sensitivity (75-91%) and specificity (87-98%) for estimating ACR 4
• Both measures show similar associations with CKD complications (anemia, acidosis, hyperparathyroidism) when adjusted for GFR 6

Predictive Value for Outcomes

• ACR is superior for predicting renal events in diabetic nephropathy, with a hazard ratio of 4.36 per 1-SD increment compared to 3.02 for PCR 7
• Both ACR and PCR are good biomarkers for cardiovascular events, renal events, and mortality in the general CKD population 5
• ACR from first-morning void has the highest area under the ROC curve for predicting renal outcomes compared to other measures 7

Important Clinical Caveats

Sample Collection

• First morning urine is preferred because it correlates well with 24-hour protein excretion and has low intra-individual variability 1
• Avoid vigorous exercise within 24 hours before collection, as it can falsely elevate results 3
• Samples should be refrigerated and analyzed within 24 hours, or stored at -70°C for longer periods (never -20°C for albumin) 1, 3

Factors Affecting Results

• Urinary tract infection, fever, heart failure, and orthostatic proteinuria can all cause transient elevations 1
• Muscle mass, age, sex, and race affect creatinine excretion and thus the denominator of both ratios 1
• Confirm abnormal results with repeat testing within 3 months before making clinical decisions 3

Guideline Recommendations

• KDIGO guidelines recommend measuring albuminuria (ACR) rather than total protein for CKD evaluation and management 1
• UNOS policy requires measurement of both ACR and PCR for living kidney donor evaluation, recognizing that non-albumin proteinuria may indicate underlying tubular disease 1
• The terms "microalbuminuria" and "macroalbuminuria" are no longer recommended; use A1, A2, A3 categories instead 1