Cystatin C (finger-prick)

Cystatin C is a small protein that almost all cells make at a fairly constant rate. The kidneys filter it from the blood. If the kidneys work less well, cystatin C rises. Unlike creatinine, which comes from muscle tissue, cystatin C hardly depends on muscle mass and meat consumption. From the value, alone or together with creatinine, the laboratory can calculate an estimated kidney function (eGFR).

• Reduced kidney function, also in people in whom creatinine gives a distorted picture, such as with a lot of or little muscle mass
• Confirming or ruling out a slightly low eGFR based on creatinine

With a raised value, the GP repeats the kidney function test and has albumin in the urine measured. If the abnormality lasts longer than three months, the approach of the Dutch GP guideline (NHG) on chronic kidney damage follows, with attention to blood pressure, medicines and the risk of cardiovascular disease; with a strongly reduced or rapidly declining kidney function, referral to an internist or nephrologist follows.

Cystatin C is a reliable kidney function measure that is less affected by muscle mass, age and diet than creatinine. According to the guideline on chronic kidney damage, an estimate based on cystatin C, or on creatinine and cystatin C together, is preferred with a creatinine-based eGFR between 45 and 60 without other signs of kidney damage, diabetes or high blood pressure, if the diagnosis has consequences. The Dutch GP guideline (NHG) on chronic kidney damage bases the classification on the eGFR and albumin in the urine. The measurement method strongly determines the result, which is why reference values differ by laboratory.

• Thyroid disorders, corticosteroids such as prednisone, smoking, overweight and inflammation affect the value without kidney function changing.
• The result differs by measurement method; preferably compare consecutive results from the same laboratory.
• One abnormal value is not a diagnosis: according to the NHG guideline, chronic kidney damage requires an abnormality that lasts at least three months.
• An acute illness or dehydration can temporarily lower kidney function.

Chronic kidney damage is common, especially at older ages: among people over 65, more than 1 in 5 have an eGFR below 60. It is often a mild, stable decrease without symptoms. How often cystatin C is abnormal in people without known kidney damage is not recorded separately.

The result says the most with a slightly low creatinine-based eGFR where there is doubt, and in people in whom creatinine gives a distorted picture: a lot of or very little muscle mass, malnutrition, an amputation, a vegetarian diet or the use of creatine. Kidney function can also be monitored in diabetes, high blood pressure or cardiovascular disease. In people without symptoms or risk factors, an abnormality is often a mild or temporary decrease.

• A slight rise due to the thyroid, prednisone, smoking or inflammation can wrongly be read as kidney damage.
• A mild, stable decrease in kidney function at an older age can cause unnecessary worry.

• NHG-Standaard Chronische nierschade (M109)
• FMS Richtlijnendatabase, richtlijn Chronische nierschade: Bepaling van nierfunctie en albuminurie
• FMS Richtlijnendatabase, richtlijn Chronische nierschade: Samenwerking met laboratoriumspecialisten klinische chemie
• NVKC, allesovertesten.nl: Cystatine C
• Thuisarts.nl: Ik heb nierschade