Albumin-creatinine ratio (ACR) in urine

The albumin-creatinine ratio (ACR) is a urine test that shows how much of the protein albumin gets into the urine compared with the amount of creatinine in the same sample. Healthy kidneys work as a filter that keeps albumin in the blood and lets only small amounts of it through into the urine. When the kidney filter is damaged, albumin starts to leak into the urine — at first in small amounts (this is called microalbuminuria), later in larger ones. The creatinine level in the sample is used as a reference point, because it reflects how concentrated the urine is — this avoids misleading results caused by the urine being more dilute or more concentrated on that day. A single urine sample (usually a morning one) is enough for the ACR test, which makes it more convenient than a 24-hour urine collection. This test matters especially for people with diabetes or high blood pressure, because it can detect kidney damage before symptoms appear and before standard blood tests change.

• In people with type 1 or type 2 diabetes — for regular monitoring of kidney function, because diabetes is the most common cause of chronic kidney disease
• In people with arterial hypertension — to assess the risk of kidney damage
• When there is a family history of chronic kidney disease
• When chronic kidney disease is suspected for other reasons — traces of protein found in a urinalysis
• To follow the course of an already known kidney disease over time

A raised albumin-creatinine ratio shows that the kidney filter is letting through more protein than usual — the earliest known marker of kidney damage. The most common cause is diabetes; it can also be high blood pressure, inflammatory kidney diseases, or a temporary rise due to fever, dehydration or physical exertion.

Causes of a raised result

• Diabetes — the most common cause, especially when it has lasted a long time or blood glucose is poorly controlled
• Arterial hypertension
• Immune or inflammatory kidney diseases
• Inherited kidney disorders
• Some cancers
• Narrowed arteries feeding the kidneys
• Systemic inflammation, fever
• Intense physical exertion or dehydration at the time of the test — a temporary rise

• Whether the test was repeated on another day, avoiding exertion and fever — a one-off rise does not always mean disease
• Blood glucose and HbA1c, if diabetes is present
• Blood pressure control
• eGFR and creatinine in the blood for an overall assessment of kidney function
• When to repeat the test to follow the change over time

ACR 30–300 mg/g — category A2

According to the KDOQI (National Kidney Foundation) eGFR calculator guidance, an albumin-creatinine ratio from 30 to 300 mg/g of creatinine is marked as category A2 — a moderately increased degree of albuminuria. This category is used together with eGFR in the KDIGO chronic kidney disease risk table: the higher the albuminuria category, the greater the need for monitoring, regardless of what the eGFR value is.

A single result in this range does not necessarily mean chronic kidney disease — the guidance states that such a diagnosis requires a raised ACR that persists for at least three months. That is why the test is usually repeated, especially if there was a fever, a urinary tract infection, intense physical exertion or marked fluid loss before the sample was taken — these factors can temporarily raise the result without reflecting ongoing kidney damage.

A doctor usually assesses this value together with eGFR, blood pressure and, if diabetes has been diagnosed, glucose and HbA1c — no single indicator on its own confirms or rules out a stage of chronic kidney disease.

• Whether the test was repeated a few weeks later, avoiding fever, infection and intense physical exertion before the sample was taken
• eGFR and creatinine in the blood — for an overall assessment of kidney function alongside the albuminuria category
• Blood pressure readings
• Glucose and HbA1c, if diabetes has been diagnosed
• When and how often to repeat the ACR test to follow the change over time

ACR above 300 mg/g — category A3

In the KDOQI eGFR calculator guidance, a result above 300 mg/g of creatinine is marked as category A3 — a severely increased degree of albuminuria. This is the highest of the three albuminuria categories (A1, A2, A3) used together with the eGFR categories (G1–G5) in the KDIGO chronic kidney disease risk table — in that table category A3 is linked with more frequent monitoring, regardless of what the eGFR value is.

A result like this usually prompts a broader work-up — creatinine in the blood, eGFR and a urinalysis to look for possible blood or other proteins are assessed, and possible causes are looked for, such as long-standing or poorly controlled diabetes, high blood pressure or inflammatory kidney diseases.

As in the other categories, a single result is not a diagnosis — the guidance provides for a repeat test, and the final conclusion about the stage of chronic kidney disease and the next steps is made by a doctor, after weighing how the results change over time and the other tests.

• Whether the test was repeated to confirm the result
• eGFR and creatinine in the blood — to establish the final stage of chronic kidney disease
• A urinalysis and the possible underlying causes (diabetes, high blood pressure, inflammatory kidney diseases)
• Blood pressure control
• How often to follow the change in this value over time