Creatinine & eGFR

What is it?

Creatinine is a normal waste substance produced when muscles are used and part of their tissue breaks down; normally the kidneys filter it from the blood and it leaves in the urine (according to MedlinePlus).

The estimated glomerular filtration rate (eGFR) is calculated by an equation from the blood creatinine level with age, sex and other factors, and shows how much blood the kidneys filter per minute (according to MedlinePlus). The CDC describes GFR as a blood test that measures how well the kidneys remove waste, toxins and excess fluid.

An alternative called measured GFR (mGFR) uses the injection of a filtration marker; MedlinePlus describes it as complex and not widely available.

Why is it done?

According to MedlinePlus, it is requested to check kidney health, to help diagnose kidney disease, to monitor existing kidney problems, to detect the effects of some drugs on the kidneys, and to check that the kidneys are healthy before treatments that may harm them.

MedlinePlus notes that risk factors that call for testing include diabetes, high blood pressure, a family history of kidney or heart disease, smoking, obesity and older age. The NIDDK states that anyone with diabetes is advised to have kidney testing annually, as is anyone with high blood pressure, heart disease or a family history of kidney failure.

Symptoms that may prompt testing include swelling of the hands or feet or puffy eyelids, fatigue, a change in how often you urinate, bloody or foamy urine, loss of appetite, muscle cramps, nausea and vomiting, and shortness of breath (MedlinePlus).

The NHS notes that people taking long-term medicines that may affect the kidneys (such as lithium, omeprazole or non-steroidal anti-inflammatory drugs) are among those who are tested regularly.

Preparation

Your provider may ask you to avoid meat for 24 hours before a creatinine test, and you may need to fast for up to 12 hours if creatinine is part of a comprehensive panel (MedlinePlus). For an eGFR test, you may be asked to fast or avoid certain foods such as meat for several hours.

Tell the medical team about all the medicines and supplements you take, as they may affect the result (MedlinePlus).

How is it done?

Blood test: a healthcare worker draws a sample from a vein in the arm with a small needle, and the process usually takes less than five minutes (MedlinePlus).

Urine test (if requested): usually a 24-hour urine collection, in which the first urination is discarded and the time recorded, then all subsequent urine is collected in the container provided and kept refrigerated until the last urination after 24 hours, then returned to the clinic or laboratory (MedlinePlus).

In measured GFR (mGFR), a marker is injected and then a blood sample is taken every 2 to 3 hours (MedlinePlus).

Understanding the result

MedlinePlus does not give reference numbers on its creatinine page, and stresses that the value depends on muscle mass, diet, age and activity. The number is therefore interpreted with your laboratory's report and your doctor's interpretation.

A raised creatinine may be linked to kidney disease or injury (infection, reduced blood flow to the kidneys, urinary tract obstruction, kidney failure) or to conditions that affect the kidneys such as heart failure and diabetes. MedlinePlus cautions that it does not always mean a kidney problem, as it can be caused by dehydration, muscle disorders and injuries, intense exercise or a meat-rich diet.

A low level may be linked to malnutrition or to conditions that weaken muscle or cause muscle loss with age, and may also occur in severe liver disease (MedlinePlus).

eGFR ranges as given in MedlinePlus: 90 to 100 makes kidney disease unlikely; 60 to 90 may indicate early kidney disease; 15 to 60 may indicate kidney disease; below 15 may indicate kidney failure. The page did not state the unit; the NHS states that eGFR is measured in ml/min. Laboratories and guidelines differ, so what counts is your laboratory's report and your doctor's interpretation.

The NIDDK and CDC state: a GFR of 60 or more is within the usual range, below 60 may mean kidney disease, and 15 or less (NIDDK) or below 15 (CDC) is described as kidney failure. The NHS gives stages by eGFR: over 90, 60 to 89, 45 to 59, 30 to 44, 15 to 29, and below 15 ml/min, and says healthy kidneys should filter more than 90 ml/min. These are guideline thresholds and not a diagnosis in themselves.

Risks and side effects

According to MedlinePlus, the risks of a blood draw are very small: mild pain or a bruise at the needle site that usually goes away quickly. A urine test has no risks. Marker substances for mGFR may cause an allergic reaction, but this is uncommon.

Limits of the test

Creatinine may stay within the usual range in the early stages of kidney disease and then rise as it progresses (MedlinePlus), which is why it is often supplemented by a urine albumin test.

Creatinine alone is limited because its production varies with muscle, diet, age and activity, and MedlinePlus describes eGFR as more accurate than creatinine alone. Cystatin C can be used instead because it is not affected by muscle size, age or diet.

After the test

When a result is abnormal, further kidney tests may be requested, such as a urine protein test and imaging (MedlinePlus). The NIDDK and CDC note that the albumin test may be repeated once or twice to confirm.

Common questions