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Lab test update - ACR

The albumin-creatinine ratio has become the proteinuria test of choice in new NICE CKD guidance. Chemical pathologist Dr Stuart Smellie uses a primary care case history to illustrate its use

The albumin-creatinine ratio has become the proteinuria test of choice in new NICE CKD guidance. Chemical pathologist Dr Stuart Smellie uses a primary care case history to illustrate its use

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What is ACR and what does testing it involve?

The ACR is the urine albumin-to-creatinine ratio. It is the test now recommended by NICE to detect raised kidney protein loss, which was historically expressed as grams (or milligrams) of protein per 24 hours or alternatively as milligrams per mmol of creatinine in urine.

It involves collecting a urine sample – preferably the first of the day – and sending it to a lab for the test to be done.

But before anything else, you should trace his last renal function measurement to see if the eGFR figure of 47 is a large drop from previously. If a value six months before had been, say, 65, I would seek nephrology advice.

How accurate are ACR results and should they be repeated?

We used to measure protein loss on a 24-hour urine sample, though this was notoriously unreliable because of inaccurate – usually incomplete – collections. The ACR is a more accurate – but not perfect – estimator of 24-hour urine protein losses because albumin is the most abundant protein and can be measured more accurately than total urine protein at lower levels. It would be sensible to perform these annually in patients with chronic kidney disease (CKD) stage 3 and beyond and in patients at risk of developing CKD.

Reagent dipstick results are quite specific in that they don't generate many false positives. But they are relatively insensitive, missing between 20% and 60% of ACR over 30mg/mmol depending on the study, and can be associated with operator error. NICE rightly recommends that if they are used, the result should be based on two out of three measurements.

How should the results be interpreted and how do they influence management in a case like this?

In diabetic patients, ‘microalbuminuria' – actually meaning raised urine albumin loss below the traditional reagent strip detection limit – is defined as 2.5mg/mmol in women or 3.5mg/mmol in men. It is a risk factor used to indicate the need for renal and cardiac preventive treatments.

In non-diabetic patients, an ACR of 30mg/mmol with haematuria, or 70mg/mmol without haematuria, is considered a prompt to seek further advice. The NICE guidance is that the threshold of 70mg/mmol is a signal to start ACE inhibitors, even if the patient is not diabetic or hypertensive. These two thresholds equate to urine protein losses of about 0.5g and 1g respectively.

The value of 30mg/mmol is approximately what would produce positive on a reagent strip.

Why is ACR being suggested in favour of the more traditional reagent strips or the protein-creatinine ratio?

Conventionally a 24-hour protein measurement was requested if the patient had dipstick-positive proteinuria on a reagent strip used in the surgery. These are not bad tests at all, but miss a proportion of cases because of, for example, dilute urine making the protein concentration undetectable. The lab-calculated ratio against urine creatinine partially allows for this.

When the 24-hour protein measurement was replaced in most routine use by the albumin or protein to creatinine ratio, some labs did the PCR and some did the ACR as there was no real evidence of which was best.

In reality, although the values differed the tests seemed pretty well equivalent. The albumin method is also more expensive. But ACR is now considered to be the better of the two tests.

We do not know much about the long-term implications of low-level proteinuria in many non-diabetic patient categories – and in fact even the definition of proteinuria varies considerably between organisations.

Is the test widely available? If not, what should be used instead?

Almost all labs measure urinary albumin because they measure microalbuminuria for diabetic patients, although their methods may not be ideal for albumin concentrations approaching 100mg/mmol. It does make sense for labs to measure the same thing if for no other reason than to reduce potential for confusion, although the change may take some time.

The table below shows approximate equivalences for the various ways of testing for proteinuria.


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Dr Stuart Smellie is consultant chemical pathologist for County Durham and Darlington Acute Hospitals NHS Trust, and associate director of the specialist interests section of the Association for Clinical Biochemistry

Competing interests: None declared

Measuring proteinuria in CKD patients now involves sending a sample to the lab Measuring proteinuria in CKD patients now involves sending a sample to the lab case Tests

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