Creatinine test

Overview

A creatinine test is a measure of how well the kidneys are doing their job of filtering waste from the blood.

Creatinine is a chemical compound left over from energy-making processes in muscles. Healthy kidneys filter creatinine out of the blood. Creatinine exits the body as a waste product in urine.

A measurement of creatinine in your blood or urine helps your healthcare professional find out how well your kidneys are working.

Why it's done

Your healthcare professional may order a creatinine test for the following reasons:

• To find out if kidney disease is the cause of symptoms you've been having.
• To check for kidney disease if you have diabetes, high blood pressure or other conditions that raise the risk of kidney disease.
• To find out if existing kidney disease is getting worse or to track how well treatment is working.
• To watch for side effects of medicines, including kidney damage or a change in kidney function.
• To closely watch how well a transplanted kidney is working.

How you prepare

A standard blood test helps measure creatinine levels in blood, also called serum creatinine levels. Your healthcare professional may ask you not to eat overnight before the test. This is called fasting.

For a creatinine urine test, you may need to collect urine over 24 hours in containers that the clinic gives you.

For either test, you may have to stop eating meat for a set amount of time before the test. If you take a creatine supplement, you'll likely need to stop use. Creatine is a compound that's used for reasons such as building muscle mass. It can affect creatinine levels.

What you can expect

A serum creatinine test involves getting a blood test. You have a needle inserted into a vein in your arm and a sample of blood is taken.

For a urine test, you provide a single sample of urine in the clinic. Or you collect samples at home over 24 hours and return them to the clinic.

Results

Healthcare professionals measure creatinine in blood or urine and interpret the test results in many ways, including the following:

Serum creatinine level

Creatinine usually enters the bloodstream and is filtered from the bloodstream at a rate that tends to be constant. The amount of creatinine in blood should be somewhat stable. A rise in the level of creatinine may be a sign that the kidneys aren't working as they should.

Serum creatinine is reported as milligrams of creatinine to a deciliter of blood (mg/dL). Or it's reported as micromoles of creatinine to a liter of blood (mmol/L). The typical range for serum creatinine is:

• For adult men, 0.74 to 1.35 mg/dL (65.4 to 119.3 mmol/L).
• For adult women, 0.59 to 1.04 mg/dL (52.2 to 91.9 mmol/L).

Glomerular filtration rate, also called GFR

The measure of serum creatinine also can be used to estimate how quickly the kidneys filter blood. This estimate is called the glomerular filtration rate. Serum creatinine varies from one person to another, so the GFR may provide a more accurate reading on kidney function.

To figure out GFR, a healthcare professional takes into account serum creatinine count and other factors such as age and sex. A GFR score below 60 suggests kidney disease. The range of scores below 60 can help track treatment and gauge whether kidney disease is becoming worse.

Creatinine clearance

Creatinine clearance is a measure of how well the kidneys filter creatinine out of the bloodstream. Creatinine leaves the body in urine after it's filtered from blood.

Healthcare professionals usually measure creatinine in a 24-hour urine sample and in a serum sample to figure out your creatinine clearance. The urine and blood samples are taken during the same time period. But shorter time periods for urine samples may be used. Accurate timing and collection of the urine sample is important.

Your healthcare professional reviews your test results with you. Results lower than the typical range for your age group may mean that the kidneys aren't working as they should.

Albumin/creatinine ratio

Another method for healthcare professionals to interpret urine creatinine count is called the albumin/creatinine ratio. Albumin is a protein in blood. Healthy kidneys don't tend to filter it out of the blood. So there should be little to no albumin in the urine.

Albumin/creatinine ratio describes how much albumin is in a urine sample in relation to how much creatinine there is. The results are reported as the number of milligrams (mg) of albumin for every gram (g) of creatinine. Results that suggest a healthy kidney are:

• For adult men, less than 17 mg/g.
• For adult women, less than 25 mg/g.

A higher than typical result may be a sign of kidney disease. The result may mean that diabetes has damaged the kidneys and led to a condition called diabetic nephropathy. This also is known as diabetic kidney disease.

Your healthcare professional talks with you about the results of a creatinine test. Ask what the results mean for your health or treatment plan.