Cystatin C

Cystatin C is a small protein that almost every cell in your body makes at a fairly constant rate. It inhibits certain enzymes, but that is not why it is measured. It is measured because your kidneys filter it almost completely out of the blood and break it down: if the kidneys filter less, it accumulates. The amount in your blood is therefore a mirror of your filtration. Creatinine does the same in principle, but creatinine comes from muscle. Anyone with a lot of muscle mass, a strength athlete or a large man, makes more creatinine and therefore appears to have worse kidney function than they have. Anyone with little muscle, a slight or older woman, someone who lost a lot of weight, makes little and gets an estimate that is too rosy. Cystatin C does not have that problem, and that is the whole reason it exists. Your report shows cystatin C in mg/L, often with an eGFR beside it calculated from it. The range rises with age, because kidney function declines slowly from your thirties; one lab builds that into age bands, from about 0.60 to 1.03 mg/L in a man of thirty to 0.82 to 1.53 mg/L over seventy, where the other uses one range of 0.62 to 1.11 mg/L for all adults.

Your kidneys are silent workers: they filter some 180 litres of blood a day, and you only notice something once a large part of their capacity is gone. A kidney function that declines slowly, from years of high blood pressure, high blood sugar or excess weight, gives no symptoms until it is far along. That is why a kidney value is on almost every report, and why it matters that the value is right. That is exactly where cystatin C earns its place for anyone who wants to stay healthy and exercises a lot. A strength athlete with a high creatinine gets an eGFR that looks too low, and a second measurement on cystatin C removes the worry. Conversely, someone with little muscle can have a normal creatinine while the kidneys are already giving ground; cystatin C does show that. In general practice it is used for that reason when the eGFR from creatinine cannot be trusted, and to confirm or dismiss a suspicion of kidney damage. The best estimate comes from both together. Where the two agree, they confirm each other; where they diverge, the gap itself is the information, and it almost always points at muscle mass or at one of the other things that move cystatin C itself. Because cystatin C is not entirely independent of who you are either. An overactive thyroid, prednisone, smoking, excess weight and inflammation raise it a little without the kidneys changing; an underactive thyroid lowers it. Small effects, but worth knowing when the two estimates diverge.

A high cystatin C means, first of all, less filtration, and the usual causes of that are the familiar ones: years of high blood pressure, diabetes, excess weight, smoking, and age itself, because from your thirties filtration declines slowly. It rises temporarily with dehydration and with medicines that burden the kidneys, such as anti-inflammatory painkillers like ibuprofen and naproxen. And without the kidneys changing: an overactive thyroid, prednisone, smoking and excess weight. A low cystatin C is favourable: your kidneys filter briskly and little stays behind. A deficiency of cystatin C does not exist. Occasionally a low value is an underactive thyroid, which slows production; TSH shows that. Protecting your kidney function is what protects your blood pressure, blood sugar and weight: losing weight around the middle, daily movement, little salt, not smoking and drinking enough. Anti-inflammatory painkillers such as ibuprofen and naproxen are the medicines that most often burden healthy kidneys; use them briefly and not when you are dehydrated. Anyone who already has kidney damage can slow the decline but not reverse it, and then blood pressure and blood sugar are the two that count most. Read cystatin C beside creatinine and the eGFR from both, and beside protein in the urine, which shows damage earlier than filtration does. When comparing over time, keep to one and the same lab, because one compares your value with your age group and the other with all adults.