High creatinine: what it means, common causes and what happens next

Creatinine is a waste product made by your muscles in the normal course of living. The kidneys filter it out of the blood and it leaves in the urine. A blood test measures how much is in the blood at that moment. If the kidneys are not clearing it as well as usual, the level rises.

The NHS says that acute kidney injury is diagnosed using a blood test in which your blood is checked for levels of creatinine, and that higher levels than usual mean the kidneys are not working as well as they should. For chronic kidney disease the NHS calls the creatinine blood test the main test.

Because creatinine comes from muscle, a result has to be read with the person in mind. That is why the lab and your doctor use your age, size and gender to turn the figure into an eGFR, and why the same creatinine can mean different things in different people.

The NHS says a doctor uses your blood test result, plus your age, size and gender, to calculate how many millilitres of waste your kidneys should be able to filter in a minute, the eGFR. Healthy kidneys should be able to filter more than 90 ml/min, and you may have chronic kidney disease if your rate is lower than this. A urine test is also done to check albumin and creatinine in the urine, known as the albumin:creatinine ratio (ACR), and to check for blood or protein.

The NHS gives the eGFR as a stage from 1 to 5 and the ACR as a stage from A1 to A3. For both, a higher stage indicates more severe kidney disease. A single raised creatinine does not on its own give a stage; the stage depends on persistent results alongside signs of kidney damage.

A high creatinine is a sign that something is affecting kidney function, not a diagnosis in itself. The NHS lists causes of acute kidney injury including infections or sepsis, dehydration, low blood pressure or blood loss, conditions that cause swelling or a blockage such as kidney stones or an inflamed prostate, long-term conditions affecting the kidneys, heart or liver, and certain medicines such as ACE inhibitors, anti-inflammatory medicines (NSAIDs) and diuretics. The dye used for some scans can also be a cause. It adds that AKI is more common if you are aged 65 or older, have had AKI before, have recently had surgery or take illegal drugs.

For chronic kidney disease the NHS lists high blood pressure, diabetes, high cholesterol, kidney infections, glomerulonephritis, polycystic kidney disease, blockages in the flow of urine and the long-term regular use of certain medicines such as lithium and NSAIDs.

Some results are only mildly raised and are explained by dehydration, a recent illness or a medicine, and settle when repeated. Others reflect longer-term kidney problems. The pattern over time is what helps to tell them apart.

The NHS says there are usually no symptoms of kidney disease in the early stages, which is why it may only be found when a blood or urine test is done for another reason. At a more advanced stage, symptoms can include tiredness, swollen ankles, feet or hands, shortness of breath, feeling sick and blood in the urine.

For acute kidney injury, the NHS says many people do not get symptoms, especially if it is mild, but that symptoms may include peeing a lot less than usual, feeling very thirsty, feeling or being sick, swollen feet or legs, feeling tired, dizzy, confused or sleepy and feeling breathless, especially when lying down. It advises an urgent GP appointment or NHS 111 if you think you may have symptoms of acute kidney injury.

A GP will usually look at the result in context, which means checking your symptoms, medicines, hydration, blood pressure and previous results. The NHS says if a doctor thinks you have AKI they may also ask for a urine sample, ask how often you have been peeing and check your pulse and blood pressure, and may arrange an ECG or scans such as an X-ray, CT scan or ultrasound.

For possible chronic kidney disease, the NHS says a urine test for ACR and blood or protein is done alongside the eGFR, and that other tests may include an ultrasound, MRI or CT scan to see what the kidneys look like and check for blockages, or a kidney biopsy. The NHS notes that if AKI is severe or the cause is not clear you may be referred to a kidney specialist (nephrologist) or admitted to hospital.

Repeat testing is a normal part of the process, because a single result may reflect a temporary change. Ask which medicines you should continue and which to pause until the repeat result is known, rather than stopping prescribed medicines on your own.

The NHS says AKI is a serious condition that can be life-threatening if not treated, but many people recover fully within a few weeks with no long-term effects, and that some people end up with permanent damage that causes chronic kidney disease. For chronic kidney disease, the NHS says it is a long-term condition that can get worse over time, that many people live long lives with it, and that it progresses to kidney failure in around 2 in 100 people with the condition. Healthy lifestyle changes and medicines to control blood pressure and cholesterol help to protect the kidneys and reduce the risk of cardiovascular disease.

The NHS suggests that if you have had AKI or are at higher risk, you can lower your chances by staying hydrated, so that your pee is pale and regular through the day, and by speaking to a doctor before taking any new medicines or supplements.

Creatinine is part of routine NHS blood testing and results are usually available through your GP surgery and, in many areas, your online GP health record. Private blood tests are widely available and include kidney profiles, but a private result still needs to be interpreted with the rest of your history and, if abnormal, repeated and followed up through your GP. Avoid changing medicines or fluid intake dramatically on the basis of a single result.