Kidney Function Test (KFT)
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Overview
A Kidney Function Test (KFT) — also called Renal Function Test (RFT) — is a blood test that measures how well your kidneys are working. The kidneys filter waste from the blood; when they are not working well, that waste accumulates. KFT is essential for anyone with diabetes, high blood pressure, or known kidney problems — and is a standard part of most routine health check-ups.
What is this test?
A KFT typically measures: creatinine (a waste product cleared by the kidneys), blood urea or BUN (another waste product), and electrolytes — sodium, potassium. eGFR (estimated Glomerular Filtration Rate) is calculated from creatinine and gives the most useful measure of kidney function.
Why your doctor may order it
- Monitoring kidney function in diabetes and hypertension
- Investigation of swelling in the legs, face, or eyes
- Suspected dehydration
- Suspected kidney disease — fatigue, change in urine, foamy urine, blood in urine
- Before starting or while on certain medications (NSAIDs, some antibiotics, contrast dye)
- Before surgery or before a contrast scan
- Annual health check-up — particularly over 40, or with family history of kidney disease
- Monitoring known chronic kidney disease
How long does it take?
The blood draw itself takes 2 to 5 minutes.
How to prepare
- Fasting is not always needed for KFT alone. If it is combined with a lipid panel or fasting glucose, fast for 8 to 12 hours.
- Stay well-hydrated in the day before — drink water normally. Don't deliberately drink large amounts just before the test.
- Avoid heavy exercise for 24 hours — strenuous activity can raise creatinine temporarily.
- Tell us about all medications and supplements — many affect kidney values.
- Avoid high-protein meals (especially red meat) the day before — they can transiently raise creatinine.
- Bring your prescription, ID, and previous KFT reports for comparison.
What to expect during the test
A blood sample is taken from a vein in your arm — usually 2 to 5 ml. The needle is in for a few seconds and the sample is collected into one. If a urine sample is needed, you will be given a clean container. For a routine sample, the middle part of the urine stream is collected (“midstream”). For a 24-hour collection (used in some cases), you will be given detailed instructions in advance.
After the test
- Resume normal activities.
- Drink water normally for the rest of the day.
- Reports are typically available the same day.
- Discuss the report with your treating doctor — small changes are common, and the trend over time matters more than a single number.
Do's and Don'ts
- Stay well-hydrated in the day before the test.
- Tell us about all medications, especially painkillers and any newly started drugs.
- Bring previous KFT reports for trend comparison.
- Mention any history of kidney disease, kidney stones, or diabetes.
- Provide a midstream urine sample if asked — first few drops into the toilet, then collect.
- Don't have a large red meat meal the day before — it can temporarily raise creatinine.
- Don't do intense exercise within 24 hours of the test.
- Don't take painkillers (NSAIDs like ibuprofen) before the test unless absolutely needed — they affect kidney function.
- Don't ignore a small but persistent rise in creatinine — chronic kidney disease often progresses silently.
- Don't assume a one-off abnormal result means kidney disease — but do get a repeat and a doctor's review.
Understanding your results
Normal creatinine is roughly 0.7 to 1.3 mg/dL in adults (slightly lower in women and the elderly). Normal urea is 7 to 20 mg/dL. eGFR above 90 mL/min is considered normal; 60 to 89 is mildly reduced; 30 to 59 is moderately reduced; below 30 needs specialist nephrology input. Trend is more important than a single value. A creatinine drifting upwards over months is more concerning than a one-off mildly high value. Your doctor will interpret the result in context
Frequently asked questions
eGFR — estimated Glomerular Filtration Rate — is the best single number for kidney function. It is calculated from your creatinine, age, sex, and ethnicity. A higher eGFR is better; values below 60 sustained over time indicate chronic kidney disease.
Many reasons — dehydration, recent intense exercise, high-protein meal, certain medications (painkillers, certain antibiotics, contrast dye), or genuine kidney disease. A single value doesn't tell the full story. A repeat after correcting the obvious factors is the next step.
Yes — KFT is essential for monitoring dialysis patients, though the interpretation is different and is handled by the nephrology team.
It helps you stay well-hydrated, which is good. But artificially overloading with water just before the test is not necessary and can briefly affect electrolytes.
At least once a year, alongside a urine test for protein. More often if any abnormality is detected or if there are other kidney risk factors.