Creatinine test

Overview

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

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

A measurement of creatinine in your blood or urine provides clues to help your doctor determine how well the kidneys are working.

Why it's done

Your doctor or other health care provider may order a creatinine test for the following reasons:

• To make a diagnosis if you have signs or symptoms of kidney disease
• To screen for kidney disease if you have diabetes, high blood pressure or other conditions that increase the risk of kidney disease
• To monitor kidney disease treatment or progression
• To monitor for side effects of drugs that may include kidney damage or altered kidney function
• To monitor the function of a transplanted kidney

How you prepare

A standard blood test is used to measure creatinine levels in your blood (serum creatinine). Your doctor may ask you not to eat (fast) overnight before the test.

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

For either test, you may need to avoid eating meat for a certain period before the test. If you take a creatine supplement, you'll likely need to stop use.

What you can expect

For a serum creatinine test, a member of your health care team takes a blood sample by inserting a needle into a vein in your arm.

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

Results

Results from creatinine in blood or urine are measured and interpreted in many ways, including the following:

Serum creatinine level

Creatinine usually enters your bloodstream and is filtered from the bloodstream at a generally constant rate. The amount of creatinine in your blood should be relatively stable. An increased level of creatinine may be a sign of poor kidney function.

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

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

Glomerular filtration rate (GFR)

The measure of serum creatinine may also be used to estimate how quickly the kidneys filter blood (glomerular filtration rate). Because of variability in serum creatinine from one person to another, the GFR may provide a more accurate reading on kidney function.

The formula for calculating GFR takes into account the 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 may be used to monitor treatment and disease progression.

Creatinine clearance

Creatinine clearance is a measure of how well the kidneys filter creatinine out of the bloodstream for excretion in urine.

Creatinine clearance is usually determined from a measurement of creatinine in a 24-hour urine sample and from a serum sample taken during the same time period. However, shorter time periods for urine samples may be used. Accurate timing and collection of the urine sample is important.

Creatinine clearance is reported as milliliters of creatinine per minute per body surface area (mL/min/BSA). The typical range for men, 19 to 75 years old, is 77 to 160 mL/min/BSA.

The typical range, by age, for creatinine clearance in women is as follows:

• 18 to 29 years: 78 to 161 mL/min/BSA
• 30 to 39 years: 72 to 154 mL/min/BSA
• 40 to 49 years: 67 to 146 mL/min/BSA
• 50 to 59 years: 62 to 139 mL/min/BSA
• 60 to 72 years: 56- to 131 mL/min/BSA

Standard measures have not been determined for older adults.

Results lower than the typical range for your age group may be a sign of poor kidney function or conditions that affect blood flow to your kidneys.

Albumin/creatinine ratio

Another interpretation of urine creatinine count is called the albumin/creatinine ratio. Albumin is a protein in blood. Healthy kidneys generally don't filter it out of the blood, so there should be little to no albumin found in the urine.

Albumin/creatinine ratio describes how much albumin is in a urine sample relative 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 indicating 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. In particular, the result may indicate a complication of diabetes called diabetic nephropathy, or diabetic kidney disease.

Your doctor or other health care provider will discuss the results of a creatinine test with you and help you understand what the information means for a diagnosis or treatment plan.