Do I have to fast before I go for GFR test?

Q. Do I have to fast before I go for GFR test?

Answered by
Dr. Choudhary Devendra
and medically reviewed by iCliniq medical review team.
This is a premium question & answer published on Jun 02, 2018 and last reviewed on: Jul 01, 2023

Hi doctor,

I am a 27 year old female. For a year, I am advised to take one Sompraz D20 on an empty stomach in the morning, and one Libotryp tablet at bedtime for TB. As a part of routine check up, I want to get tested for kidney function, that is, GFR blood test. Is fasting required for this test? If so, for how long? Do I need to give my urine sample also along with blood? Please provide me all the details of GFR test. My weight is 180 pounds and height is 5.4 feet. What details am I supposed to give to the lab for GFR test?



Welcome to

I appreciate your concern regarding GFR (glomerular filtration rate). If no structural abnormality of kidneys is seen on imaging studies like ultrasound KUB (kidney, ureter, and bladder), then get serum creatinine, which is an approximate reflection of renal function. A complex investigation like GFR estimation is not required. Some labs give estimated GFR based on creatinine and body weight, for which you need not fast if no other tests are planned.

Thank you doctor,

Do I need to give a urine sample for serum creatinine test or only blood sample is required? Is fasting required for serum creatinine test?



Welcome back to

A blood test is primarily done and urine tests are reserved only in cases where any abnormality is detected. Food interferes with serum creatinine levels if kidney function is compromised, so in CKD (chronic kidney disease) and diabetic patient, fasting is preferred. But a normal kidney excretes creatinine well, so this does not matter in normal subjects. So if you have no known problem previously and just testing for screening, you may go even without fasting, which we commonly do for our patients.

Hello doctor,

Thank you for replying. Do I need to provide my height and weight to the lab? Please provide me with all the details that I am supposed to give to the lab for serum creatinine blood test.



Welcome back to

The following formula is most commonly used for creatinine clearance (eGFR) estimation: The Cockcroft and Gault formula, CCr={((l 40–age) x weight)/(72xSCr)}x 0.85 (if female). Here you can see your weight will be taken into consideration along with age and serum creatinine value. Some labs use other formulas also if already known that the patient is CKD. For you eGFR, based on creatinine and weight, age is acceptable. If your eGFR is normal, then it is fine, if any abnormality is found, then I would suggest you see a nephrologist for further evaluation. I am a surgeon for kidney diseases. I would like you to get the tests done which are, Urine analysis, ultrasound KUB region, and creatinine. I hope everything to be normal, as you are asymptomatic with an old history of tuberculosis without previous evidence of an effect on the urinary system.

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