Serum creatinine is a commonly used test to evaluate kidney function. And, if the creatinine value falls within the lab reference range, then all is considered well. However, this is not so because the creatinine level is affected by age, race, gender and also the body weight. Hence, serum creatinine level alone is an unreliable marker for impaired kidney function or chronic kidney disease.
The GFR or glomerular filtration rate is the best measure to assess the function of the kidneys.
There are several ways to calculate the GFR.
The most common is Cockcroft Gault equation. It calculates creatinine clearance (mL/min) as a surrogate for GFR. It provides an estimate of GFR based on serum creatinine, age, gender and body weight.
The formula is:
Estimated or eGFR = 140 – age x body weight (in kg)/72 × serum creatinine (in mg/dL)
· A value of 60 or higher is normal (GFR decreases with age).
· A value less than 60 is low and may indicate kidney disease.
· A value of 15 or lower may mean kidney failure.
The same serum creatinine value may mean different GFR for similarly aged persons of different body weight.
· A 40 kg man, aged 40 years, with serum creatinine of 1, the GFR is 55.
· A 50 kg man, aged 40 years, with serum creatinine of 1, the GFR is 69.
· A 60 kg man, aged 40 years, with serum creatinine of 1, the GFR is 83.
· A 70 kg man, aged 40 years, with serum creatinine of 1, the GFR is 97.
· A 80 kg man, aged 40 years, with serum creatinine of 1, the GFR is 111.
If you are at increased risk for kidney disease i.e. if you have diabetes, high blood pressure, or family history of diabetes, high blood pressure or kidney disease, then calculate your GFR to know if you have kidney disease.
The GFR may show impaired kidney function, even if the serum creatinine level is in the normal range.
Another important point to take note of is that instead of looking at a single test result, one should look for trends in the results.
Always compare the results of a particular test with previous reports even when a result is normal. Results that show change over time i.e. show a rising or declining trend by 0.3 are important and should not be ignored.
Any result which has been in the low normal range for the last many years, but is now in the high normal range even though still within normal limits, should be investigated accordingly.
Padma Shri Awardee
President Elect Confederation of Medical Associations in Asia and Oceania (CMAAO)
Group Editor-in-Chief IJCP Publications
President Heart Care Foundation of India