GFR (Glomerular Filtration Rate)
- amount of filtrate formed per minute by two kidneys combined
- best test to measure the level of kidney function and determine the stage of kidney disease
- doctor can calculate it from the results of patient's blood creatinine test, patient's age, race, gender and other factors
- Normal GFR varies according to age, sex, and body size; in young adults it is approximately 120-130ml/min/1.73 m2 and declines with age.
- GFR can be expressed in ml/min or ml/min/1.73 m2
- GFR in Males approx 5mls higher, females approx 5mls lower). Therefore values >90 are normal for all. Converting for average surface area (per 1.73m2) removes the sex difference.
Attention: For clearer image of the above diagram, pls click on the picture. Creatine Clearance Test
- The creatinine clearance test compares the level of creatinine in urine with the creatinine level in the blood, usually based on measurements of a 24-hour urine sample and a blood sample drawn at the end of the 24-hour period. Clearance is often measured as milliliters/minute (ml/min).
- Because creatinine is found in stable plasma concentrations, is freely filtered and not reabsorbed, and is minimally secreted by the kidneys, creatinine clearance is used to estimate the glomerular filtration rate (GFR) -- the standard by which kidney function is assessed.
- Creatinine clearance overestimates GFR in a variety of circumstances, because of tubular secretion. This is particularly important at low levels of GFR. To calculate it, pay close attention to units and remember that there are 1440 minutes in 24hr . However 24 hour urine collections are error-prone and have been shown to be less accurate for estimating GFR than deductions from plasma creatinine.
Calculations: everyone knows it's UV/P but the units get a little confusing. This formula shows SI units.
| UV: | Amount of creatinine in 24h of urine is reported by the lab in mmol/24h. x1000 to convert to micromols |
| P: | Serum creatinine is reported in micromols/l. x1000 to do calculation for mls |
| 1440: | Number of minutes in 24h |
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How 24-hours Urine test is performed?
- On day 1, urinate into the toilet when you get up in the morning.
- Afterwards, collect all urine in a special container for the next 24 hours.
- On day 2, urinate into the container when you get up in the morning.
- Cap the container. Keep it in the refrigerator or a cool place during the collection period. Label the container with your name, the date, the time of completion, and return it as instructed.
- do not exercise more than usual while collecting urine
- if any urine is spilled and not collected, 24-hours urine test needed to be restarted.
GFR from Creatinine Clearance Test Result
- Normal values of GFR are as follows (normal value ranges may vary slightly among different laboratories):
* Male: 97 to 137 ml/min.
* Female: 88 to 128 ml/min.
Note: ml/min. = milliliters per minute
http://www.kidney.org/kidneydisease/ckd/knowGFR.cfm
http://www.nlm.nih.gov/medlineplus/ency/article/003611.htm
Contributed by Lawrence Oh
