Calculating Kidney Function

Estimated GFR Calculator
What does the ‘Estimated Kidney Function’ result mean?
What treatment should be given?

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.

Estimated GFR calculator

To use the calculator correctly, please note that:

Creatinine must be in the units micromoles per litre. The value will be roughly 60-120 for someone with good kidney function, and 350-600 for someone approaching dialysis. In some parts of the world, such as the USA, the units used are different – milligrammes per decilitre. This gives values of about 0.05 – 1.0 for normal kidney function, and 3.0 – 5.0 for someone approaching dialysis.
Weight must be in kilogrammes
Age – to the nearest year
Ethnicity – people of black African origin have, on average, a higher level of creatinine for a given body weight than people of other (white, Asian, south Asian etc) ethnicities. This can be important, because the creatinine level may appear to be high in some people of black ethnicity who actually have completely normal kidney function.

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
Plasma creatinine

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 formula was devised by Cockcroft and Gault and is discussed in the Oxford Textbook of Clinical Nephrology, and has been modified locally in the light of recent work, published in the Annals of Internal Medicine 1999; 130: 461-70. .
To use the calculator enter values for gender, ethnicity, age, weight and plasma creatinine and click on calculate. Please note that if the patient has gross oedema or is very malnourished, then the calculator may still overestimate the creatinine clearance.

What does the ‘Estimated Kidney Function’ result mean?

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
Over 75	Normal kidney function. There could be some kidney disease present, but at the present time there is no serious damage to the kidneys
40-75	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.
15-40	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
Over 75	60-120
40-75	110-200
15-40	175-350
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.

What treatment should be given?

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
Everyone	healthy diet stop smoking lose weight treat high blood pressure
40-75	As above plus:- usually nothing additional
15-40	As above plus:- generally need to see kidney specialist see what potassium levels are from blood tests and diet if needed see what phosphate levels are and diet/treat if needed (link to section) see what haemoglobin level is and treat if anaemic (Click here for more information on anaemia)
Less than 15	As above plus:- need a plan for dialysis or transplant, or to maintain without dialysis usually need to see specialist renal dietitian

NKF Controlled Document No. 286, Calculating Kidney Function, written 6 October 2004. Last reviewed 28 May 2009.

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.

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Page created: 6 October 2004

Last updated: 29 April 2009

Gender (M/F)
Black ethnicity? (Y/N)
Age in years
Weight (kg)
Plasma creatinine µmol/l

Estimated GFR, ml/min: