Albumin-to-Creatinine Ratio (uACR) Test: Staging Kidney Damage From One Cup of Urine

One raised ratio is not a diagnosis. Chronic kidney disease requires a problem that lasts more than three months, and urine albumin varies a lot from day to day. KDIGO advises confirming a ratio of 30 mg/g or more from a random sample with a first-morning, midstream sample, which is less affected by the day's activity and fluids. For screening, MedlinePlus describes two more tests within three to six months, with early kidney disease considered when two of the three are abnormal.

Several things can raise a single result for a short time:

So a first result of 48 mg/g handed in the morning after a long run, or during a period, calls for a repeat. For the repeat, ask whether a first-morning sample is wanted, keep the day before quiet, and mention a period, fever or urinary symptoms. A home sample should go back promptly; it keeps for days in the fridge, but freezing breaks down albumin and makes the result read low.

KDIGO stages chronic kidney disease on two measures together: filtration, from the eGFR blood test, and albuminuria, from the uACR. Risk is read where the two meet:

The top row is where people are most often caught out. An eGFR of 95 is normal filtration, but with a confirmed uACR of 60 the combination counts as moderately increased risk. Albumin in the urine can signal kidney disease while eGFR is still above 60, which is why both tests are ordered. eGFR comes from blood creatinine, a different measurement from the urine creatinine in this ratio. The guide to reading a kidney panel explains how the two results are read together on a kidney panel.

Albumin concentration on its own depends on how much you drank. Creatinine passes into urine at a fairly steady rate, so dividing by it cancels most of the dilution, which lets a single spot sample stand in for a 24-hour collection.

The cost is that creatinine output depends on muscle mass and diet, and anything that moves it moves the ratio. Women excrete less creatinine than men, so they read a higher ratio for the same albumin leak, and low body weight raises the ratio too. A high-protein diet and exercise raise creatinine output and push the ratio down, so a very muscular person can get a reassuring ratio from a leak that would be flagged in a smaller person. That is why you may be asked to avoid meat the day before the sample.

Because people with early albumin leakage feel nothing, the test only helps as a routine check. The NIDDK advises yearly kidney testing for everyone with type 2 diabetes and for people who have had type 1 diabetes for more than five years, which is why the ratio is also part of a diabetes panel.

The National Kidney Foundation also recommends at least yearly testing for people with high blood pressure, heart disease or heart failure, a family history of kidney disease, a body mass index over 30, those who smoke, and those over 60. Once a result is abnormal, rechecks range from yearly to four or more times a year, depending on the ratio, the eGFR and whether kidney disease has been diagnosed.

Diabetes is the leading cause of kidney disease, and long-standing high blood pressure causes similar damage over years. Together they account for most persistent albuminuria; diseases of the kidney filters, lupus and some medicines account for much of the rest. Heavy, ongoing loss eventually shows in the blood as a falling serum albumin. Where diabetes or blood pressure is the cause, kidney protection comes mainly from controlling them, including blood sugar.

Once kidney disease is known, the question becomes whether the leak is growing. Because albumin output varies so much, KDIGO treats a doubling of the ratio on a later test as exceeding the usual variation and warranting evaluation. A rise from 45 to 100 mg/g clears that bar. A rise from 45 to 70 may still be variation, so it is judged by whether it holds on repeat samples. The direction over several samples, as the guide to tracking lab trends explains, matters more than any single result.