What is the albumin‑creatinine ratio (ACR), what are the normal and abnormal values, and how should an elevated ACR be confirmed and managed in patients at risk for chronic kidney disease?

Albumin-Creatinine Ratio: Definition, Values, and Clinical Management

What is the Albumin-Creatinine Ratio?

The albumin-creatinine ratio (ACR) is a urine test that measures kidney damage by comparing the amount of albumin (a protein) to creatinine in a single urine sample, eliminating the need for cumbersome 24-hour urine collections. 1, 2 The ratio corrects for variations in urine concentration due to hydration status, making it far more convenient and accurate than timed collections. 1, 3

Normal and Abnormal ACR Values

The KDIGO 2024 guideline establishes the following categories for ACR interpretation: 1

• Normal (A1): ACR <30 mg/g (<3 mg/mmol)
• Moderately increased albuminuria (A2): ACR 30-299 mg/g (3-30 mg/mmol)
• Severely increased albuminuria (A3): ACR ≥300 mg/g (≥30 mg/mmol)

The term "microalbuminuria" is outdated and should no longer be used. 2 Some studies suggest sex-specific cutoffs (>17 mg/g in men, >25 mg/g in women) due to differences in creatinine excretion, though the 30 mg/g threshold remains the standard recommendation. 1

Important Caveat on ACR Interpretation

In individuals with obesity, the standard 30 mg/g threshold may underestimate albuminuria because higher muscle mass increases urinary creatinine excretion, artificially lowering the ACR relative to actual albumin loss. 4 Clinicians should maintain heightened suspicion in obese patients with "borderline" values.

How to Properly Test for ACR

Optimal Collection Method

Obtain a first morning void midstream urine sample—this is the gold standard with the lowest coefficient of variation (31%) and best reproducibility. 1, 2, 5 If a first morning sample is unavailable, collect samples at the same consistent time of day. 5

Pre-Collection Instructions

Before collecting urine, patients must: 1, 5

• Avoid vigorous exercise for 24 hours (exercise falsely elevates albumin excretion)
• Fast for at least 2 hours prior to collection
• Not be menstruating (menstruation falsely elevates ACR)
• Not have active urinary tract infection, fever, or acute illness

Laboratory Handling Standards

Laboratories must: 1

• Analyze samples fresh or store at 4°C for maximum 7 days
• Never freeze samples at -20°C (this compromises albumin measurement accuracy)
• Report ACR to one decimal place (not albumin concentration alone)
• Maintain analytical coefficient of variation <15%
• Participate in external quality assessment programs

Confirmation Protocol for Elevated ACR

Because day-to-day ACR variability can exceed 80% even in stable patients, a single elevated result is insufficient for diagnosis. 6 The confirmation algorithm is: 1, 5

1. If initial ACR ≥30 mg/g: Obtain 2 additional first morning void samples over the next 3-6 months
2. Diagnosis requires: At least 2 out of 3 samples showing ACR ≥30 mg/g
3. Before confirming, exclude transient causes: 1, 2, 5
   • Active urinary tract infection or fever
   • Congestive heart failure exacerbation
   • Marked hyperglycemia
   • Menstruation
   • Uncontrolled hypertension (>160/100 mmHg)
   • Recent vigorous exercise

Management of Confirmed Elevated ACR

For ACR 30-299 mg/g (Moderately Increased Albuminuria)

Immediately initiate an ACE inhibitor or ARB regardless of baseline blood pressure, as these agents provide kidney-protective effects beyond simple blood pressure lowering. 1, 2 Target blood pressure <130/80 mmHg. 1, 2

Additional interventions include: 2

• Optimize glycemic control (HbA1c <7% in most diabetic patients)
• Restrict dietary protein to 0.8 g/kg/day
• Achieve LDL cholesterol <100 mg/dL (diabetic) or <120 mg/dL (non-diabetic)
• Limit saturated fat to <7% of total calories

ACE inhibitors and ARBs are absolutely contraindicated in pregnancy and women of childbearing potential not using reliable contraception due to teratogenic effects. 2

Monitoring Frequency Based on eGFR and ACR

The KDIGO 2024 guideline specifies: 1, 2

eGFR (mL/min/1.73 m²)	ACR 30-299 mg/g	ACR ≥300 mg/g
≥60	Annually	Every 6 months
45-59	Every 6 months	Every 3-4 months
30-44	Every 3-4 months	Every 3 months
<30	Immediate nephrology referral	Immediate nephrology referral

A doubling of ACR on subsequent testing exceeds laboratory variability and warrants immediate evaluation for disease progression. 1

For ACR ≥300 mg/g (Severely Increased Albuminuria)

This represents advanced kidney damage with very high cardiovascular and progression risk—nephrology referral is mandatory. 2 In addition to ACE inhibitor/ARB therapy and blood pressure control, these patients require: 1, 2

• Monitoring every 3-6 months depending on eGFR
• Assessment using kidney failure risk equations (e.g., KFRE) to determine timing of kidney replacement therapy planning
• Aggressive cardiovascular risk factor modification

When to Refer to Nephrology

Immediate nephrology referral is indicated for: 1, 2

• eGFR <30 mL/min/1.73 m² at any ACR level
• ACR ≥300 mg/g persistently despite therapy
• Rapid progression (eGFR decline >20% or ACR doubling on subsequent testing)
• Refractory hypertension requiring ≥4 antihypertensive agents
• Uncertainty about underlying etiology of kidney disease
• Inadequate response to optimal ACE inhibitor/ARB therapy

Screening Recommendations

For diabetes: Begin screening at diagnosis for type 2 diabetes (disease onset uncertain) or 5 years after diagnosis for type 1 diabetes, then annually thereafter. 2, 5 For hypertension and other high-risk conditions: Annual screening is recommended. 1, 5

Point-of-Care Testing Considerations

Point-of-care ACR devices may be used when laboratory access is limited, but must detect at least 85% of patients with ACR ≥30 mg/g to be acceptable. 1 Semiquantitative dipstick tests have poor sensitivity (43.6%) and high false-discovery rates for detecting ACR ≥30 mg/g—any positive dipstick result must be confirmed with quantitative laboratory ACR measurement. 7