Albumin-to-Creatinine Ratio (Spot)
ACR· also: Urine ACR, Albumin/Creatinine Ratio, UACR, Spot ACR
Clinical Overview
The Albumin-to-Creatinine Ratio (ACR), measured on a spot (random) urine sample, is the preferred test for detecting and quantifying urine albumin excretion in clinical practice. By dividing albumin by creatinine (both in the same sample), ACR corrects for urine dilution — providing a reliable estimate of 24-hour albumin excretion without the inconvenience and inaccuracy of a 24-hour urine collection.
Why This Test Matters
ACR is the most important test for detecting early kidney disease in diabetes and hypertension. Microalbuminuria (ACR 30–300 mg/g, formerly called "microalbuminuria") is the earliest detectable sign of diabetic nephropathy and indicates glomerular endothelial damage. It is also a powerful independent risk factor for cardiovascular disease. ACR is a key component of the CKD staging system (along with eGFR): CKD categories A1 (ACR <30), A2 (ACR 30–300), and A3 (ACR >300). Higher ACR categories predict faster kidney function decline and higher cardiovascular risk. Interventions that reduce ACR — ACE inhibitors, ARBs, SGLT-2 inhibitors, blood pressure control, and glycemic control — reduce the risk of progression to end-stage kidney disease. ACR should be measured annually in all patients with diabetes, hypertension, or other CKD risk factors.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 0 – 30 | mg/g | Normal (A1 category) |
| 30 – 300 | mg/g | Moderately increased (A2) — formerly microalbuminuria |
| 300 – 9999 | mg/g | Severely increased (A3) — formerly macroalbuminuria |
Also reported in: mg/mmol.
What Causes Abnormal Results?
High ACR Causes
- Diabetic nephropathy (most common cause of elevated ACR)
- Hypertensive kidney disease
- Glomerulonephritis (any cause)
- Lupus nephritis
- Pre-eclampsia in pregnancy
- IgA nephropathy
- Focal segmental glomerulosclerosis (FSGS)
- Heart failure (cardiorenal syndrome)
- Transient elevation: exercise (collect specimen on non-exercise day), fever, urinary tract infection
Low ACR Causes
- ACR <30 mg/g is normal — represents healthy glomerular filtration barrier function
Signs & Symptoms to Watch For
How to Prepare for This Test
A first morning midstream urine sample is preferred (minimizes the effect of orthostatic proteinuria and exercise). Collect on a day without vigorous exercise. If ACR is elevated, confirm on 2 of 3 samples collected over 3 months — transient elevations are common. Avoid testing during UTI or fever (inflammatory states transiently raise ACR).
Factors That Can Affect Results
- Vigorous exercise within 24 hours falsely raises ACR — always test on rest day
- UTI and fever transiently raise ACR — retest after resolution
- Highly concentrated or dilute urine samples affect ratio accuracy (extreme hydration or dehydration)
- Orthostatic proteinuria (benign, mainly in young adults) — collect first morning specimen lying flat to avoid
- Menstruation — blood contamination raises protein; avoid collection during period
Related Topics
Frequently Asked Questions
My ACR is 45 mg/g — what does this mean and what should I do?
An ACR of 45 mg/g falls in the A2 category (moderately increased albuminuria, formerly called microalbuminuria). This means your kidneys are leaking a small but significant amount of albumin and indicates early kidney damage, most commonly from diabetes or high blood pressure. Action steps: (1) Confirm with a repeat ACR on 2 more occasions over 3 months — transient elevations occur. (2) Optimize blood pressure to <130/80 mmHg using an ACE inhibitor or ARB (specifically kidney-protective in CKD with proteinuria). (3) If diabetic, tighten glucose control (HbA1c target <7%). (4) Consider SGLT-2 inhibitor (finerenone or empagliflozin) — proven to reduce ACR and slow CKD progression. (5) Nephrology referral if ACR remains above 300 or kidney function is declining.
Why is the ACR better than a 24-hour urine protein collection?
24-hour urine protein collection is cumbersome and often inaccurate because patients frequently mistime or under/over-collect. The ACR on a spot sample corrects for urine concentration by dividing albumin by creatinine — since creatinine excretion is relatively constant throughout the day, this ratio approximates daily albumin excretion accurately. Studies have validated that spot ACR correlates closely with 24-hour albumin excretion, making it the preferred clinical test. 24-hour urine protein collection is still used for nephrotic syndrome evaluation and monitoring response to treatment when precise quantification matters.