How to Calculate Glomerular Filtration Rate

The glomerular filtration rate (GFR) is a measure of how well your kidneys are functioning

Glomerular filtration rate (GFR) is a measure of how well your kidneys are working. Your GFR depends on your age, gender, race and the amount of creatinine in your blood. Creatinine, a waste material produced by using the muscles in your body, is removed by the kidneys. The better your kidneys are functioning, the lower your serum creatinine levels will be and the higher your GFR will be. Ideally, your glomerular filtration rate should be above 60.

Instructions

1. As we age, our kidney function declines, and the amount of creatinine in our blood increases and GFR decreases. Because creatinine levels increase with increasing muscle mass, men have higher creatinine levels than women, and African-Americans, because of their larger body types, have higher levels of creatinine than other races. Equations for calculating GFR use specific multipliers to adjust for the effects age, sex and race have on creatinine levels.

2. Choose the correct equation to calculate your GFR. If you're a mathematician, the National Institutes of Health’s National Kidney Disease Education Program provides two equations for calculating GFR (for adults over 18), depending on whether or not creatinine measurements were obtained using an IDMS traceable reference standard. IDMS stands for isotope dilution mass spectrometry, the gold standard for determining creatinine levels.

   For creatinine levels obtained not using an IDMS reference method:

   GFR (mL/min/1.73 m2) = 186 x (Scr)-1.154 x (Age)-0.203 x (0.742 if female) x (1.212 if African American)

   For creatinine levels obtained using an IDMS reference method:

   GFR (mL/min/1.73 m2) = 175 x (Scr)-1.154 x (Age)-0.203 x (0.742 if female) x (1.212 if African American).

   where Scr = serum creatinine, mL = milliliters, min = minutes, m2 = meters squared (a measure of body surface area).

3. Choose an online GFR calculator if you don't want to do the math yourself. The National Kidney Foundation, The National Kidney Disease Education Program (NIH) and DaVita websites all provide fill-in-the-blanks GFR calculators (see References). For the first two sites, you will need to know your creatinine level and units of measurement, either mg/dL or μmol/L. DaVita requires creatinine levels to be reported in mg/dL. Select the correct unit of measurement before you type in your creatinine level. Then add your age, race and gender. The calculator will determine your GFR. DaVita allows you to track sequential GFR readings and store your results online.

Tips & Warnings

• One GFR reading is rarely definitive. Your doctor will monitor your kidney function by measuring your creatinine levels periodically, particularly as you age. As you reach your retirement years, or if you are having kidney problems, your doctor will probably monitor your kidney function at least on an annual basis.

• Only your doctor can tell you what your specific GFR results mean.

References

• National Kidney Foundation: Glomerular Filtration Rate: GFR
• National Kidney Disease Education Program: Health Professionals
• DaVita GFR Calulator