eGFR, weight and blood sugar: what counts?

The eGFR value comes from an equation rather than a measurement. The lab measures your creatinine, a waste product of muscle tissue that your kidneys remove from your blood, and calculates the CKD-EPI result from it together with your age and your sex: how many millilitres of blood your kidneys would filter per minute at a standard body surface area of 1.73 square metres. Some labs also report an EKFC result: two equations, the same starting point, a slightly different number.

That conversion to a standard body deserves attention at a high body weight. The value says what your filtration would be at an average build, not what your kidneys actually process per minute. For choosing drug doses your doctor sometimes uses the non-converted value, and in a large body that difference is anything but small.

The equation has one blind spot that comes into play while losing weight: it assumes everyone produces creatinine at the same rate. If your muscle mass or your protein intake changes, your creatinine changes with it and so does the calculated eGFR, without your kidneys having started to work differently. Cystatin C, a protein that does not come from muscle, sidesteps that problem.

With excess weight and in the early phase of diabetes the kidney often filters harder than normal, and that is called hyperfiltration. On paper it produces a nice high value. In reality the filter units work under raised pressure, and over time that can actually damage the kidney. An eGFR above 90 is therefore no proof of healthy kidneys; many labs simply report "greater than 90" above that, because the equation is imprecise there.

Albumin in the urine often shows damage earlier than the eGFR does. That is why, with excess weight or a raised HbA1c, your doctor requests both tests: one says how hard the kidney filters, the other whether the filter is already leaking.

The ordinary reading still applies. Above 90 is normal, 60 to 89 is called mildly reduced, and below 60 only counts once it persists for three months or protein is present in the urine. The value also falls on its own by about 1 ml/min per year after forty.

While losing weight you look mainly at the direction. Blood pressure and blood sugar usually improve as your weight falls, and for the kidney that is what counts. The eGFR itself can meanwhile move a few points because your muscle mass and protein intake change; so compare measurements under comparable conditions and with your own baseline, and do not read a single shift as gain or loss.

With raised weight or disturbed blood sugar, kidney function belongs in the picture, and the eGFR is usually calculated automatically as soon as creatinine is measured. Ask about albumin in the urine too, as it completes the picture.

Measure once before you start losing weight or start GLP-1 medication, so you have your own baseline. During substantial weight loss many things change at once: muscle mass falls and protein intake shifts, and both affect creatinine and therefore the calculated value. So compare measurements under comparable conditions: on an ordinary day, well hydrated, not right after hard exertion and without a large meat meal the night before.

If the result does not fit your situation, cystatin C can help, because it does not depend on muscle mass. One value under 60 is not a diagnosis; only after three months of the same result is there chronic kidney damage. Discuss any abnormal value with your doctor.