Creatinine is produced by the body’s natural processes of breaking down muscle cells and rebuilding them. Any one person will produce the same amount of creatinine each day. Creatinine is filtered out of the bloodstream by the kidneys. The level the creatinine builds up to in the bloodstream therefore depends on a balance between the amount produced and the efficiency of filtration in the kidneys.
The problem with using creatinine levels to assess kidney function is that the total amount of creatinine produced each day depends on how much muscle the person has, and people vary a lot in how much muscle they have. In general, women have less muscle than men of a similar age, and older people have less muscle than younger people. People of African origin also tend to produce more creatinine than people of white European descent. This means that a given level of creatinine in the blood can indicate normal kidney function in one person, but only 30% of normal kidney function in another. Doctors therefore should take age, gender, and weight into account when interpreting serum creatinine levels, but this isn’t easy.
One way of getting round this problem is to measure the amount of creatinine a person produces in a day, as well as the level in the blood, by asking the person to collect all the urine they pass in a 24-hour period. However, these 24-hour urine collections are a nuisance to do, and it’s easy to forget to collect urine each time you empty the bladder – so these measurements can be wrong.
For more information on how blood tests to measure creatinine levels can be used to estimate the level of kidney function in an individual, click here.
The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.