Understanding Proteinuria, Albuminuria and ACR: What Do Your Urine Kidney Results Actually Mean?

When people are monitoring kidney disease, most attention tends to go to blood results such as creatinine and estimated glomerular filtration rate (eGFR). However, urine testing provides a different and equally important piece of information about kidney health.

In particular, protein or albumin leaking into the urine can indicate damage to the kidney’s filtering structures and provides important information about the risk of chronic kidney disease (CKD) progressing. Albuminuria may also be present when eGFR is still relatively normal, which is why kidney disease assessment should generally consider both blood and urine results.

Like eGFR, however, the albumin ratio (ACR) is not completely stable. A result can change quite substantially between tests without necessarily meaning that your kidneys have suddenly deteriorated or improved. The pattern over time is generally more informative than an isolated result.

What is proteinuria?

Proteinuria means that an abnormally high amount of protein is being lost into the urine.

Healthy kidneys continuously filter the blood through microscopic structures called glomeruli. Water and small molecules pass through this filtration barrier, while most larger proteins are retained within the circulation.

If this barrier becomes damaged or abnormally permeable, proteins can pass into the urine.

Importantly, proteinuria does not simply mean that protein you have eaten has passed through your kidneys. Dietary protein can influence kidney haemodynamics in some circumstances, but significant persistent proteinuria generally reflects abnormalities in kidney filtration or protein handling.

Proteinuria is a broad term because urine can contain several types of protein. Albumin is the one most commonly measured when assessing CKD.

What is albuminuria?

Albumin is the main protein circulating in the blood. An increased amount of albumin in the urine is called albuminuria.

Albuminuria commonly occurs when the glomerular filtration barrier has been damaged. This can occur in diabetic kidney disease, hypertension and glomerular disorders such as IgA nephropathy, focal segmental glomerulosclerosis and other forms of glomerulonephritis.

Proteinuria and albuminuria therefore overlap, but they are not exactly the same thing. Albuminuria refers specifically to albumin, whereas proteinuria includes albumin and other urinary proteins.

This distinction occasionally matters. Some tubular kidney disorders and conditions involving abnormal proteins, such as monoclonal light chains, can cause proteinuria that is not predominantly albumin. In these situations, tests other than ACR may be required.

For routine CKD assessment, however, ACR is generally the preferred test because even relatively small increases in albumin excretion provide useful information about kidney and cardiovascular risk (KDIGO 2024; Kidney Health Australia 2024).

What does an ACR test actually measure?

ACR stands for albumin ratio.

Rather than simply measuring how much albumin is present in a urine sample, the laboratory compares urinary albumin with urinary creatinine.

This helps compensate for how concentrated or dilute the urine happens to be.

For example, if you are dehydrated, a urine sample may be considerably more concentrated. Conversely, a large intake of fluid can produce very dilute urine. Measuring albumin concentration alone could therefore give quite different results depending upon hydration.

Using the albumin ratio makes a single urine sample much more useful and avoids the need for most people to undertake a 24-hour urine collection.

When practical, a first-morning urine sample is preferable because it reduces some of the variation caused by physical activity, posture and time of day (KDIGO 2024; Kidney Health Australia 2024).

What do the ACR numbers mean?

In Australia, ACR is usually reported in mg/mmol.

Current KDIGO and Kidney Health Australia classifications divide albuminuria into three categories (KDIGO 2024; Kidney Health Australia 2024):

CategoryUrine ACRInterpretationA1<3 mg/mmolNormal to mildly increasedA23–30 mg/mmolModerately increasedA3>30 mg/mmolSeverely increased

Kidney Health Australia’s fifth-edition CKD handbook uses the same thresholds (Kidney Health Australia 2024).

You may still see the older terms microalbuminuria and macroalbuminuria on pathology reports. These correspond broadly with A2 and A3 respectively, although the newer terminology is preferable.

The categories are useful, but it is also important to consider the actual number. An ACR of 4 mg/mmol represents a very different degree of albumin leakage from an ACR of 200 mg/mmol, even though both are abnormal.

ACR and eGFR measure different aspects of kidney health

This is perhaps the most important concept when interpreting these results.

eGFR estimates the kidney’s filtration function.

ACR provides information about damage to the filtration barrier and the amount of albumin leaking through it.

They are related, but they are not measuring the same thing.

Someone can therefore have an eGFR above 90 but persistent albuminuria for more than three months and still meet the criteria for CKD. Conversely, someone can have substantially reduced eGFR with relatively little albuminuria.

This is why modern CKD classification combines eGFR and albuminuria rather than relying on either result alone. As albuminuria rises, the risk of CKD progression and cardiovascular disease increases at any given eGFR (KDIGO 2024; Kidney Health Australia 2024).

An eGFR of 50 with an ACR of 1 mg/mmol therefore represents a different clinical picture from an eGFR of 50 with an ACR of 150 mg/mmol.

Why can ACR change between tests?

Patients sometimes become understandably concerned when an ACR that was previously 10 suddenly returns at 18 or 25 mg/mmol.

However, urinary albumin excretion has considerable biological variability.

Exercise, acute illness, infection, blood pressure, blood glucose, hydration, heart failure, urinary tract infection, menstruation and blood within the urine can all influence results. The timing of urine collection can also matter.

This means a small rise or fall should not automatically be interpreted as progression or recovery.

KDIGO (2024) specifically advises that, for people with CKD, a doubling of ACR on a subsequent test exceeds the degree of variability ordinarily expected and warrants evaluation.

That does not mean smaller movements are irrelevant. A pattern such as:

10 → 18 → 27 → 45 mg/mmol

over repeated comparable samples is clearly different from:

22 → 17 → 25 → 19 mg/mmol.

The first suggests a sustained upward trajectory. The second may simply represent fluctuation around a reasonably stable baseline.

This is similar to interpreting eGFR: individual numbers matter, but the longer-term trajectory usually tells us more.

What happens if ACR is elevated for the first time?

A single abnormal ACR does not necessarily establish chronic kidney disease.

Kidney Health Australia (2024) recommends repeating an ACR of 3 mg/mmol or above within three months. If the repeat result is normal, a third sample should be obtained, preferably using a first-morning urine. At least two elevated results over three months support persistent albuminuria.

The circumstances surrounding testing should also be considered. An elevated result collected while someone has a urinary infection, acute illness or following vigorous exercise may need to be repeated once that factor has resolved.

If albuminuria persists, the question becomes why.

Blood pressure, blood glucose, kidney function, urine microscopy, medications and the possibility of an underlying glomerular disorder may all need consideration. Marked proteinuria, blood in the urine, declining kidney function or other features of glomerular disease may warrant further nephrology investigation.

Can albuminuria improve?

Yes, and this is an important distinction from thinking about CKD simply in terms of lost filtration capacity.

The amount of albumin leaking through the kidneys is partly influenced by structural kidney damage, but it is also affected by pressure within the glomeruli and other potentially modifiable factors.

Blood pressure is particularly important. Excessive pressure within the glomerular circulation increases stress on the filtration barrier and can increase albumin leakage.

This is one reason medications that inhibit the renin–angiotensin system, including ACE inhibitors and angiotensin receptor blockers, are commonly used in albuminuric CKD. Their renal effects extend beyond simply reducing the blood pressure measured on your arm (KDIGO 2024).

SGLT2 inhibitors have also become an important treatment for many people with CKD because of their effects on glomerular haemodynamics and their demonstrated ability to reduce the risk of kidney disease progression in appropriate patients (KDIGO 2024).

Other considerations can include diabetes management, sodium intake, smoking, body weight and treatment directed at the underlying kidney disease. The appropriate approach therefore depends considerably on why the albuminuria is occurring.

Does reducing albuminuria actually matter?

Persistent reductions in albuminuria are generally considered favourable, particularly when baseline albuminuria is substantial.

A meta-analysis of randomised controlled trials by Heerspink et al. (2019) found that treatment-related reductions in albuminuria were associated with reduced risk of subsequent major kidney outcomes.

Similarly, an individual participant-level meta-analysis by Coresh et al. (2019) found that reductions in albuminuria were associated with a lower subsequent risk of end-stage kidney disease across a large and diverse population.

This does not mean that every 20 or 30% reduction on a single pathology result represents a comparable improvement in kidney health. ACR naturally fluctuates, so sustained changes across multiple measurements are much more meaningful.

How often should ACR be monitored?

Monitoring frequency depends upon the type and severity of kidney disease.

Someone with stable kidney function and A1 albuminuria may require relatively infrequent testing. Someone with A3 albuminuria, progressive CKD or an active glomerular disease may require substantially closer monitoring.

Testing may also be repeated after introducing treatments intended to reduce albuminuria.

Kidney Health Australia (2024) recommends regular Kidney Health Checks in at-risk populations, while monitoring in people with established CKD should be individualised according to eGFR, albuminuria, underlying diagnosis and risk of progression.

The aim is not to test so frequently that normal biological variation becomes a source of unnecessary concern. It is to test often enough to identify a genuine change in the trajectory.

What about foamy urine?

Proteinuria can cause noticeably foamy or frothy urine, particularly when protein loss is substantial.

However, the appearance of urine is not a reliable measure of albuminuria.

Concentrated urine, the force of the urinary stream and even cleaning products in the toilet can alter how foamy urine appears. Some people with significant proteinuria notice obvious frothing, while others notice very little.

If proteinuria is a concern, a laboratory measurement is much more informative than trying to judge it visually.

Looking at ACR in context

ACR is one of the most useful markers we have for assessing kidney disease, but it should not be interpreted in isolation.

When reviewing kidney results, I generally want to understand the relationship between:

eGFR and creatinine + ACR or proteinuria + blood pressure + the underlying kidney disease + the direction of change over time

A small change in ACR between two tests does not necessarily indicate that kidney disease has changed. Equally, a stable eGFR should not automatically be considered reassuring if albuminuria is progressively increasing.

For people monitoring CKD, the objective is therefore not necessarily to achieve exactly the same ACR every time. It is to identify whether persistent albuminuria is present, understand why it is occurring, reduce it where possible and monitor the longer-term trajectory.

As with eGFR, one result is a snapshot. The pattern over time provides considerably more information.

References

Coresh, J, Heerspink, HJL, Sang, Y et al. 2019, ‘Change in albuminuria and subsequent risk of end-stage kidney disease: an individual participant-level consortium meta-analysis of observational studies’, The Lancet Diabetes & Endocrinology, vol. 7, no. 2, pp. 115–127, doi:10.1016/S2213-8587(18)30313-9.

Heerspink, HJL, Greene, T, Tighiouart, H et al. 2019, ‘Change in albuminuria as a surrogate endpoint for progression of kidney disease: a meta-analysis of treatment effects in randomised clinical trials’, The Lancet Diabetes & Endocrinology, vol. 7, no. 2, pp. 128–139, doi:10.1016/S2213-8587(18)30314-0.

Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group 2024, ‘KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease’, Kidney International, vol. 105, no. 4S, pp. S117–S314, doi:10.1016/j.kint.2023.10.018.

Kidney Health Australia 2024, Chronic Kidney Disease (CKD) Management in Primary Care, 5th edn, Kidney Health Australia, Melbourne.