Cystatin C is a novel marker of –
**Core Concept**
Cystatin C is a protein that is produced by all nucleated cells and serves as a cysteine protease inhibitor. It is a novel marker used to estimate kidney function, particularly in patients with chronic kidney disease (CKD) or those on the extremes of body weight, where serum creatinine may not be accurate.
**Why the Correct Answer is Right**
Cystatin C is an endogenous compound that is freely filtered by the glomerulus and neither secreted nor reabsorbed by the renal tubules. Its levels are not affected by age, sex, muscle mass, or diet, making it an excellent marker for estimating glomerular filtration rate (GFR). The Cystatin C-based eGFR (eGFRcys) is calculated using the following formula: eGFRcys = (k * cystatin C) / (1 + (k * cystatin C)), where k is a constant. This value is then used to assess kidney function and monitor the progression of CKD.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as cystatin C is not a marker of liver function.
Cystatin C is a marker of kidney function, not liver function. Liver function is typically assessed using other markers such as albumin, bilirubin, or prothrombin time.
**Option B:** Incorrect, as cystatin C is not a marker of cardiac function.
Cystatin C is not directly related to cardiac function, although CKD is a significant risk factor for cardiovascular disease.
**Option C:** Incorrect, as cystatin C is not a marker of nutritional status.
Cystatin C is a marker of kidney function, not nutritional status, which is typically assessed using other markers such as albumin, prealbumin, or transferrin.
**Clinical Pearl / High-Yield Fact**
Cystatin C is a valuable marker in patients with CKD, as it can detect mild renal impairment and monitor the progression of kidney disease. It is also useful in patients with obesity or muscle wasting, where serum creatinine may not accurately reflect kidney function.
**Correct Answer:** C.