Kidney Corner: Breaking down your GFR

Dr. Mark Saddler
Ace Stryker | Southern Ute Drum

Dr. Mark Saddler“My doctor says my GFR is down. What does this mean and what should I do about it?”

GFR, which stands for “glomerular filtration rate,” is one of the most useful ways of measuring kidney function. It represents the rate at which blood is filtered by the kidney and is an approximation of the ability of the kidneys to remove toxins.

When measuring the kidney’s function, it is more usually more practical to measure toxin levels in the blood than in the urine. The lower the kidney function, the higher the level of toxins in the blood.

We usually measure a substance called creatinine in the blood. It’s released from breakdown of muscle cells at a fairly constant rate and is only removed by the kidneys. So the level of creatinine has become established over the years as a reasonable measure of kidney function.

However, as might be expected, since creatinine originates in muscle, muscular people produce more creatinine than thin people — and this can cause problems. For example, a very muscular male athlete might have a higher creatinine level than, say, a thin elderly lady, even if those two individuals’ kidney function remains the same.

For this reason, we use a correction formula to determine GFR that takes into account creatinine level, age and sex. This is useful to help assess the significance of a given creatinine level.

It’s important to realize that GFR declines with age. A healthy 21-year-old person without kidney disease may have a GFR around 115 (typically a little higher in men than in women). Even healthy people, however, may deviate quite considerably from these numbers.

And the GFR measured on different days may also vary quite considerably based on the hydration status at the time of the measurement (and possibly also due to some inherent potential inaccuracy of the test, which appears to be unavoidable.) As we age, GFR tends to decline, and it’s normal for levels to decline down to the 70s, 60s and even 50s with age.

The National Kidney Foundation defines chronic kidney disease as occurring when GFR declines below 60. This has been a controversial definition, because as you can see from our preceding discussion, normal aging of the kidney could result in a person being labeled as having chronic kidney disease.

It’s possible that over time, this definition of kidney disease may change, but in the meantime it’s worth remembering that kidney function tests should be interpreted with consideration to age, and that a decrease in kidney function may be more ominous in a young than an old patient.

As we have discussed previously in the Kidney Corner, chronic kidney disease is classified according to the GFR, from Stage 1 chronic kidney disease, in which the GFR is normal, to Stage 5 chronic kidney disease, in which the GFR is below 15, around which level dialysis typically needs to be started.

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