Kidney function is estimated in the clinic by taking blood and measuring the concentration of a chemical within the blood called creatinine. This is a waste product of muscles in the body and, if the kidneys are not working well, the level of creatinine in the blood will rise.
The blood level of creatinine does not give a full measurement of kidney function, because of variations between individuals. Some clinics use urine collections or radio-isotope methods to estimate kidney function better. If your clinic uses these tests, discuss the results with the staff.
It is also possible to estimate kidney function from the blood creatinine level, and the Glomerular Filtration Rate (GFR) calculator below can be used to do this. To estimate the kidney function, enter your creatinine level, age, ethnicity and weight into the calculator.
To use the calculator correctly, please note that:
If you do not have a creatinine clearance measured by 24 hour urine collection and blood sample, creatinine clearance can be estimated from the level of creatinine in the blood. The calculator below does this for you, using the following formula:
|Creatinine clearance =||
1.2 x (140 - age in years) x weight in kg
Note: if the patient is female the result of the formula is multiplied by 0.85; if the patient is of black ethnicity the result of the formula is multiplied by 1.18.
The estimated kidney function result from the calculator above should be between 10 and 150; if it is outside these values, there has probably been a mistake entering the numbers.
|Estimated Kidney Function||Interpretation|
Normal kidney function. There could be some kidney disease present, but at the present time there is no serious damage to the kidneys
Kidney function may be slightly reduced. Though many apparently fit and healthy elderly people have levels of 40-75. This level kidney function will not cause any symptoms.
Reduced kidney function, tests to determine the exact nature of the problem with the kidneys are required. This level of kidney function, unless it improves rapidly, will be called ‘chronic kidney failure’ by the medical team. The individual, though, generally feels well in themselves.
|Less than 15||The level of kidney function is dangerously low, and dialysis may be required. There may be symptoms such as tiredness or sickness or itching. Urgent assessment by a kidney specialist is recommended if the individual has not previously been seen.|
For those not able to enter enough data into the calculator to estimate kidney function, here are the levels of creatinine in the blood that approximate to the levels of kidney function. Younger and larger people have higher creatinine levels for a given estimated kidney function.
|Estimated kidney function||Blood creatinine level, micromoles per litre|
|Less than 15||300 upwards|
If there are a series of measurements of the estimated kidney function over a period of years, the rate of change of kidney function can be assessed. In some people, even though the kidney function is reduced, it may remain stable for many years. In other cases there is a reduction in kidney function, and it may be possible to predict when the function will drop to a level of 15. Plans for dialysis or a kidney transplant can then be made in advance.
Anyone who uses the calculator to estimate their own kidney function from their blood results should discuss the results with their own medical specialist, to make sure that the result and the interpretation are appropriate to their individual case.
Treatment depends upon the needs of the individual, but some broad outlines are given in the table, according to the estimated kidney function.
|Estimated kidney function (see above)||Treatment|
As above plus:-
As above plus:-
|Less than 15||
As above plus:-
NKF Controlled Document No. 286, Calculating Kidney Function, written 6 October 2004. Last reviewed 6 June 2011.
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.