eGFR from cystatin C

Every estimate of your kidney function works the same way: take a substance the body makes at a fixed pace and the kidneys filter away, and calculate how much filtration is needed to explain the measured amount in your blood. The ordinary eGFR uses creatinine for that, the waste of your muscles. This variant uses cystatin C, a small protein that almost all cells with a nucleus make at a fixed pace and that the kidneys filter away completely. The pace of production does not depend on how much muscle you have, not on what you eat and not on creatine. That is the whole point. In an average body the two estimates come out at about the same. In a bodybuilder or a creatine user creatinine gives a too low eGFR while the kidneys are healthy, and in a slight older person or someone who has been ill for long a too high one while the kidneys already do less; cystatin C sits closer in both cases. Cystatin C has its own confounders, an overactive thyroid, prednisone, smoking and excess weight lift it, but those are different from creatinine's, and that is why the two complement each other. Your report usually does not print this estimate; you calculate it with the formula on this page, from cystatin C, your age and your sex. The result is in the same unit as your ordinary eGFR, millilitres per minute for a body of standard size, and above 60 is healthy.

For most people this estimate adds little: in an average body creatinine says the same and the ordinary eGFR is enough. It counts for anyone the ordinary eGFR misleads. Anyone with a lot of muscle or using creatine who sees an eGFR just below 60 wants to know whether that is the kidneys or the muscles, and cystatin C gives that answer without having to stop training or creatine. Anyone slight, older, or who has been ill for long and sees a reassuring eGFR wants to know whether that reassurance is real, and there cystatin C sometimes shows a kidney function that sits lower. The second reason is medication. Many medicines are excreted by the kidneys and dosed on the eGFR, and if that estimate is wrong for you, the dose is wrong too. In case of doubt a prescriber or pharmacist may use an estimate from cystatin C instead of the lab value, and then notes that on the prescription. For anyone receiving chemotherapy, antibiotics or anticoagulants dosed on the kidney, that difference can count. The honest limitation is that both estimates calculate with a standard body. With a body surface far from 1.73 square metres, roughly below 1.50 or above 2.00, they both deviate the same way: too low in a large body, too high in a small one. Cystatin C solves the muscle problem, not the build problem.

A lower estimate means less is being filtered, or the input distorts. With reduced kidney function both estimates fall, and then the question belongs with it whether it lasts three months and whether there is protein in the urine. If this estimate sits below your ordinary eGFR, look at what lifts cystatin C: an overactive thyroid, prednisone, smoking, excess weight and inflammation; and at a body much larger than average, because then the formula estimates too low. A higher estimate belongs with brisk filtration, or with the same distortion the other way. If this estimate sits above your ordinary eGFR, that is usually the muscle story: a lot of muscle or creatine lowered the ordinary eGFR, and cystatin C shows the kidneys as they are. An underactive thyroid lowers cystatin C and lifts this estimate, and a body much smaller than average does the same. Read this estimate beside your ordinary eGFR, because the comparison is the whole point, beside cystatin C itself and its confounders, and beside creatinine and your muscle mass. Do not read a stage from it: the classes belong to the estimate from creatinine, or to the combined estimate from creatinine and cystatin C together.