Albumin-to-Creatinine Ratio · What Is It, How It’s Evaluated, What It Indicates, and More

The albumin-to-creatinine ratio (ACR) is a urine test that detects the ratio of albumin and creatinine in the urine. Albumin is the most abundant protein in the blood and is synthesized by the liver. In a healthy individual, an albumin molecule is generally too large to filter through the glomeruli, which are clusters of blood vessels in the kidney responsible for filtration. However, when the kidneys are damaged, albumin can pass through the glomeruli.  

On the other hand, creatinine is a metabolic waste product that can normally be found in the urine even in healthy individuals. It’s produced from the normal breakdown of muscle tissue and the consumption of protein and is excreted at a relatively constant rate. This consistency allows the albumin-to-creatinine ratio to normalize for variations in urine concentration that can happen depending on one’s hydration status, time of day, and diet (e.g., meat, vegetarian, or vegan diets). While ACR is a stable and reliable test of kidney function that accounts for these potential variances in the amount of albumin in the body, the ACR test does have limitations (e.g., impacted by vigorous exercise, diet) that should be considered when interpreting results.  

The albumin-to-creatinine ratio is evaluated through a urine test, including a single urine sample or 24-hour urine sample. Most often a single urine sample is taken at a healthcare provider’s office or in a laboratory to measure ACR, in a test known as a spot urine albumin-creatinine ratio. A single urine sample can also be timed, like in the morning upon waking or after having not voided for 4 hours. These results provide an estimate of the amount of albumin that would be excreted in the urine during a 24-hour period.  

Another way to measure ACR is through a 24-hour urine collection. This method is the most accurate and requires the individual to collect all of their urine over a 24-hour period.  

Prior to an ACR urine test, it may be recommended to avoid intense exercise since the increase in muscle breakdown can affect test results. Additionally, some may be recommended to avoid eating meat the day before the test as meat consumption may have a small effect on urine creatinine levels. Other conditions that may affect the accuracy of the results include fever or infection at the time of testing (e.g., urinary tract infection), urinary or menstrual bleeding, or a sudden dramatic increase in blood pressure or blood glucose levels.  

For any abnormal results, the ACR may be repeated every 3 to 6 months to ensure the value indicates true renal dysfunction as opposed to situational albuminuria (i.e., in the case of intense exercise).  

The indications for evaluating the albumin-to-creatinine ratio include assessing for kidney disease in individuals who are at high risk. The population at high risk for kidney disease includes those with type 1 and type 2 diabetes mellitus. Uncontrolled or poorly controlled diabetes mellitus can result in the development of diabetic nephropathy (i.e., a deterioration in kidney function) as a result of hyperglycemia (i.e., elevated blood sugar) that increases the amount of inflammatory proteins ultimately damaging blood vessels in the kidneys. Other factors that can increase the risk for kidney disease include hypertension or high blood pressure; cardiovascular disease; a family history of kidney disease; tobacco smoking; obesity; being 50 years of age or older; and systemic diseases like lupus erythematosus. 

In individuals with risk factors for kidney disease, the ACR is typically measured each year as a screening test, and if elevated, additional tests (e.g., estimated glomerular filtration rate [eGFR]) are typically performed over a 3-month interval. Urine ACR can be measured more frequently if levels of albumin are significantly raised. Additionally, if an individual presents with symptoms of kidney dysfunction including edema (i.e., swelling), proteinuria (i.e., protein in the urine), or hematuria (i.e., blood in the urine), a urine ACR may be ordered to assess kidney function.  

Since the urine albumin-to-creatinine ratio is a general measure of kidney function, an elevated value may indicate kidney damage. ACR between 30 and 299 mg/g is considered moderately increased and ACR of 300 mg/g or higher is considered severely increased. 

ACR can also be a predictor of progression or worsening of kidney damage in someone who has already been diagnosed with chronic kidney disease. An elevated ACR is also associated with an increased risk of cardiovascular events, including myocardial infarction or heart attack; stroke; and heart failure. Additionally, not only can hypertension lead to increases in urine ACR, but an elevated ACR can predict the onset of hypertension in those who previously had normal blood pressure. In those with diabetes mellitus, an increased ACR is associated with a higher risk of developing complications secondary to the disease, such as diabetic retinopathy (i.e., growth of abnormal blood vessels in the retina); neuropathy or nerve damage; and nephropathy.   

Two elevated results for 3 months or more can be a sign of kidney disease. In addition to the urine ACR, a blood test called the estimated glomerular filtration rate (eGFR) may also be done to assess how well the kidneys are functioning. If laboratory testing is abnormal, imaging tests, such as ultrasound or computed tomography (CT) of the kidneys may be performed to further aid in diagnosing any underlying causes.