Kidney function tests

Kidney function tests, also called KFT or RFT (renal function tests), show how well your kidneys filter waste from your blood. Creatinine is a waste product from normal muscle use. Urea, reported as BUN (blood urea nitrogen), is a waste product made when the body breaks down protein. Healthy kidneys clear both, so they build up in the blood when the kidneys are not working well. Your age and sex, and sometimes your height and weight, are used with the creatinine result to work out the eGFR (estimated glomerular filtration rate), an estimate of how fast your kidneys filter. A urine test for albumin, a protein, checks whether the kidneys are leaking it, and is often reported as a ratio to creatinine (ACR).

Doctors use these tests to find kidney disease early, to see how serious it is and follow it over time, and to check whether a medicine is harming the kidneys. Early kidney disease usually has no symptoms, so testing is the only way to know. You may also be tested if you have swelling of the legs or face, changes in passing urine, itching or tiredness. Regular testing is advised if you have diabetes, high blood pressure or heart disease, a family history of kidney disease, other kidney problems such as kidney stones, or take medicines that can affect the kidneys. People with diabetes should have their kidneys checked every year.

Often no special preparation is needed, but labs differ, so ask. You may be asked to fast if creatinine or urea is part of a larger panel on the same sample. Some labs ask you not to eat meat for some hours beforehand, because meat can raise creatinine for a while.

The blood test goes like this:

1. You sit down and roll up your sleeve. A small needle goes into a vein in your arm. You may feel a brief sting.
2. Blood runs into a small tube, usually in under five minutes.
3. For the urine test, you pass a little urine into a container at the lab or clinic.
4. The samples are sent to the lab.

Some urine tests need all your urine over 24 hours. You empty your bladder once and throw that urine away, then collect every sample for the next 24 hours and keep the container cool.

A small bruise may appear where the needle went in.

A blood test carries very little risk. You may have slight pain or bruising where the needle went in, and this usually fades quickly. A urine test carries no risk.

The report lists each result next to the lab's own normal range, which depends on age, sex and the method the lab uses. Creatinine also depends on muscle size, diet and activity, so one person's normal can differ from another's. eGFR is reported as a number, and a higher number generally means healthier kidneys. Lower numbers can point to kidney disease, from early to advanced.

One abnormal result is not a diagnosis. Dehydration, a diet high in meat and hard exercise can raise creatinine, and a normal result does not always rule out early kidney disease. The doctor may repeat the test and add urine tests, an ultrasound or CT scan, and sometimes a kidney biopsy. eGFR can be unreliable in severe malnutrition, muscle-wasting conditions and advanced liver disease. Once kidney disease is found, regular tests show whether treatment is working.