All about Kidney disease tests (GFR, ACR)

Monitoring kidney function is vital for early diagnosis and treatment of renal failure, tracking kidney disease progression and adjusting medication and treatment plans in accordance to kidney function level.   

Level of kidney function is generally determined and assessed by two different tests: The first being a blood test – the Glomerular Filtration Test (GFR) and the second being a urine analysis test – the Albumin to Creatinine Ratio (ACR).

Glomerular Filtration Rate (GFR)

A blood test that measures kidney function by measuring the flow rate of filtered fluid through the kidney. The result is based on a calculation combining blood creatinine levels with age, body size and gender and determines stage of kidney disease. GFR is typically recorded in units of milliliters per minute. A declining in GFR numbers indicate a deteriorating in kidney function.

In adults, the normal GFR is more than 90, with the number declining with age:

GFR of more than 90, with evidence of kidney disease such as proteinuria, genetic diagnosis of kidney disease or abnormal kidney structure, indicates kidney damage with normal kidney function, known as stage 1 of chronic kidney disease (CKD).

GFR of 60-89 indicates kidney damage with mild loss of kidney function, stage 2 of CKD

GFR below 60 for a period of 3 months or more, or above 60 but with high levels of albumin in urine, is an indicator for kidney disease. At this point the goal is preventing further deterioration, and keeping kidney function at this level by use of medication combined with nutritional and lifestyle changes and a frequent monitoring of GFR levels:

GFR of 44-59 indicates mild to moderate loss of kidney function, stage 3a of CKD

GFR of 30-44 indicates moderate to severe loss of kidney function, stage 3b of CKD

GFR of 15-29 indicates severe loss of kidney function, stage 4 of CKD

GFR of 15 or lower is referred to as kidney failure, or stage 5 of CKD, and will require intense intervention such as dialysis or a kidney transplant.

Albumin to Creatinine Ratio (ACR)

One of the markers for kidney damage is persistent increased levels of protein in the urine, with at least two positive tests taken over a 3-month period. Albumin is the most common type of protein found in urine and testing for albumin in urine (known as albuminuria) can act as an early marker of kidney disease. Albumin is a protein present in high concentration in the blood, and not present in the urine when kidney function is normal. Kidney function deterioration prevents adequate filtering of the protein and albumin escapes from the kidneys into the urine. Albumin may be detected in urine from early stages of CKD.  Testing may be used when for screening for kidney function, in attempt to recognize early stages of kidney disease and allow for appropriate treatment.

Creatinine is a byproduct of muscle metabolism and is released into the urine at a constant rate, the creatinine measurement is used to correct for urine concentration in any urine sample.

The preferred method of testing for elevated levels of the protein in urine is using the albumin to creatinine ratio. Urine is collected (either by spot urine sample or by 24-hour collection) and ACR is calculated by dividing albumin concentration (mgs) by creatinine concentration (grams).

ACR levels lower than 30 mg/g (category A1) indicate normal to mildly increased albuminuria levels, normal kidney function.

ACR levels between 30-300 mg/g (category A2), indicate moderately increased albuminuria levels, a sign of early kidney disease.

ACR levels greater than 300 mg/g (category A3), indicate severely increased albuminuria levels, a sign of severe kidney disease.

There are a variety of condition that may result in abnormal protein levels in urine such as a urinary tract infection, blood in urine and other conditions. Testing should be repeated after condition is treated. It is recommended not to test ACR levels during acute illness or menstruation.

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