Article — ACR Calculator - Urine Albumin-Creatinine Ratio
Urine Albumin-Creatinine Ratio Calculator
The urine albumin-to-creatinine ratio (ACR) divides urine albumin (mg) by urine creatinine (g) in a single spot sample. KDIGO 2024 calls an ACR under 30 mg/g normal, 30-299 mg/g moderately increased, and 300 mg/g or higher severely increased — the bands that drive every chronic kidney disease decision.
ACR is the single most important non-invasive marker of kidney damage. Combined with estimated glomerular filtration rate (eGFR), it forms the two axes of the KDIGO CKD heat map used worldwide to grade risk and guide therapy.
What is the albumin-creatinine ratio?
Healthy glomeruli filter blood but block albumin — the most abundant plasma protein — from entering the urine. Damaged glomeruli leak albumin. Measuring how much albumin appears in urine is therefore one of the earliest signals of kidney injury.
The trick is that urine concentration varies massively with hydration. A dilute sample after a litre of water looks normal even with substantial leakage. Dividing albumin by urinary creatinine (which excretes at a steady ~1 g/day) normalizes the result. The output, ACR, is what every nephrology clinic now uses.
The National Kidney Foundation estimates 1 in 7 US adults — about 35.5 million people — has chronic kidney disease, and 90% don't know it. ACR is one of the two simplest tests (alongside eGFR) that catch the disease before symptoms.
Why ACR replaced 24-hour urine
The old standard for measuring proteinuria was a 24-hour collection — a jug carried for a full day. The method is precise in theory and unreliable in practice. Patients miss voids, exceed the collection window, or contaminate the sample. Reported errors in published studies range from 10% to over 30%.
The spot ACR removes all of that. A single urine sample takes seconds, costs less than $20 in most US labs, and correlates with 24-hour albumin excretion at r > 0.85 in validation studies. KDIGO, the American Diabetes Association and the National Kidney Foundation have all switched their guidelines to ACR.
First-morning urine gives the most reproducible ACR. Mid-day samples can be falsely elevated by exercise or postural proteinuria, especially in younger adults.
ACR formula and units
ACR (mg/g) = albumin (mg) ÷ creatinine (g)ACR (mg/mmol) = ACR (mg/g) × 0.1131 g creatinine = 8.84 mmolThe US convention reports ACR in mg/g. Most of the world uses mg/mmol — the SI form. The conversion is a constant 0.113 because urinary creatinine has a fixed molecular weight (113 g/mol). One result speaks to both audiences as long as the units are labeled clearly.
KDIGO ACR categories
- A1 = ACR under 30 mg/g (under 3 mg/mmol) — normal to mildly increased
- A2 = ACR 30-299 mg/g (3-29 mg/mmol) — moderately increased (microalbuminuria)
- A3 = ACR 300 mg/g or higher (30 mg/mmol or higher) — severely increased (macroalbuminuria)
- Nephrotic-range = ACR over 2,200 mg/g — protein loss above 3 g/day
- Daily albumin (rough) = ACR mg/g approximates mg/day (most adults excrete ~1 g creatinine/day)
- Diabetic threshold = ADA flags any ACR ≥ 30 mg/g for action
- Conversion factor = mg/g × 0.113 = mg/mmol
ACR in diabetes and hypertension
The two driving causes of CKD — diabetes and hypertension — both produce albuminuria early. The ADA recommends an annual ACR for every adult with type 2 diabetes from the time of diagnosis, and for every adult with type 1 diabetes after five years of disease. The AHA recommends similar screening for adults with hypertension and any cardiovascular risk factor.
Catching microalbuminuria (A2) matters because it is reversible. Tight glycemic control, blood pressure below 130/80 mmHg, and an ACE inhibitor or ARB can return ACR to A1 in a substantial minority of patients — and slow progression in the rest. SGLT2 inhibitors, originally developed for diabetes, have become a standard add-on for any patient with ACR over 200 mg/g because of their independent kidney-protective effect.
Causes of false ACR elevation
A single high ACR is not a CKD diagnosis. Many transient conditions raise urine albumin. This calculator and article are educational and do not replace clinical judgment. Discuss any abnormal result with your physician before changing treatment.
Vigorous exercise within 24 hours, fever, menstruation, urinary tract infection, congestive heart failure decompensation, and even brief hyperglycemia can all push ACR above 30 mg/g without true kidney damage. Postural (orthostatic) proteinuria in younger adults — albumin loss when upright but not lying flat — is another classic trap. This is why KDIGO requires two of three abnormal samples over three months before confirming the diagnosis.
How to lower a raised ACR
The interventions with the strongest evidence are blood pressure control, glycemic control in diabetes, renin-angiotensin blockade (ACE inhibitor or ARB), SGLT2 inhibition, smoking cessation, and a moderate-protein diet (around 0.8 g/kg/day, not lower). Each can drop ACR by 20-50% over months. Weight loss in overweight adults independently reduces albuminuria, even without other changes.
Repeating the ACR test
The KDIGO protocol is straightforward: confirm any A2 or A3 result with two more samples over three months. If two of three are abnormal, the patient has persistent albuminuria and meets CKD criteria when combined with reduced eGFR or structural damage. The same test then becomes the monitoring tool — repeated every six to twelve months to watch for progression or response to treatment.
Trend matters as much as level. An ACR that drops from 250 to 80 mg/g over six months after starting an ACE inhibitor is responding well, even though the absolute value is still in the A2 band. Conversely, an ACR creeping from 25 to 45 mg/g over a year is moving in the wrong direction and warrants attention long before crossing any guideline threshold. Patients on dialysis or with very low eGFR still benefit from ACR monitoring, because residual proteinuria predicts both cardiovascular mortality and the rate of progression in transplant recipients.