What is the significance of the Albumin (Microalbumin) to Creatinine ratio?

From the Guidelines

The albumin-creatinine ratio (ACR) is a crucial test for assessing kidney function, and a normal ACR is less than 30 mg/g, while 30-300 mg/g indicates microalbuminuria and greater than 300 mg/g suggests macroalbuminuria, as supported by the most recent study 1.

Key Points to Consider

• The ACR test should be performed annually for patients with diabetes, hypertension, or known kidney disease to detect early signs of kidney damage, as recommended by the 2021 standards of medical care in diabetes 1.
• For accurate results, a random spot urine sample is typically sufficient, though first-morning samples may be preferred, as they have been shown to have the lowest within-individual variation (CVi) of albumin excretion 1.
• If an elevated ACR is detected, confirmation with repeat testing is recommended, as transient elevations can occur due to fever, exercise, or urinary tract infections, and two of three specimens of UACR collected within a 3- to 6-month period should be abnormal before considering a patient to have high or very high albuminuria 1.
• The ACR is valuable because albumin leakage into urine is one of the earliest signs of kidney damage, particularly in diabetic nephropathy, allowing for earlier intervention with medications like ACE inhibitors or ARBs to slow disease progression, as demonstrated by studies on the use of these agents in patients with elevated albuminuria 1.

Importance of Regular Monitoring

• Regular monitoring of ACR helps track kidney function over time and evaluate the effectiveness of treatments aimed at preserving kidney health, and an active urinary sediment or rapidly increasing albuminuria may suggest alternative or additional causes of kidney disease, prompting referral to a physician experienced in the care of kidney disease 1.
• The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation is generally preferred for calculating eGFR from serum creatinine, and an eGFR persistently <60 mL/min/3.73 m2 is considered abnormal 1.
• The ACR is a continuous measurement, and differences within the normal and abnormal ranges are associated with renal and cardiovascular outcomes, highlighting the importance of regular monitoring and timely intervention 1.

From the Research

Albumin Creatinine Ratio

• The albumin-to-creatinine ratio (ACR) is a preferred method to quantify proteinuria, and it can be calculated from urine dipstick protein or protein-to-creatinine ratio (PCR) 2.
• A study found that the positive predictive value (PPV) of a single random abnormal urine ACR to diagnose chronic kidney disease (CKD) in patients with type 2 diabetes was 96.80% 3.
• The relationship between ACR and total protein:creatinine ratio (TPCR) is non-linear, and TPCR is highly correlated with 24-h urine protein 4.
• TPCR outperforms ACR at predicting 0.5 g/day and 1 g/day of proteinuria, and a TPCR threshold of 100 mg/mmol had a higher sensitivity but lower specificity than an ACR of 70 mg/mmol to predict 1 g/day of total proteinuria 4.
• The use of angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers (ACE/ARBs) has increased over time among adults with CKD, but it appeared to plateau after 2003 5.
• A study found that ACE inhibitor/ARB underutilization represents a significant gap in preventive care delivery for adults with hypertension and albuminuria, with an estimated 1.6 million adults not receiving ACE inhibitor/ARB therapy 6.

Calculation and Measurement

• Calculated ACR can be used for risk predictions when the measured ACR is not available, and PCR-calculated ACR may have superior performance to dipstick-calculated ACR 2.
• The diagnostic performance of ACR and TPCR varies substantially with age and gender, and should be taken into consideration when interpreting results 4.

Clinical Applications

• ACR is an important tool for assessing proteinuria in CKD, and it can be used to predict kidney failure and death in people with CKD 2.
• The use of ACE/ARBs is recommended as first-line antihypertensive therapy in the presence of albuminuria, but the utilization of these medications has not substantially changed over time 6.