Kidney function
Glomerular filtration rate (GFR), a marker of kidney health, is best measured by injecting compounds such as inulin, radioisotopes such as 51chromium-EDTA, 125I-iothalamate, 99mTc-DTPA or radiocontrast agents such as iohexol, but these techniques are complicated, costly, time-consuming and have potential side-effects.[2][3] Creatinine is the most widely used biomarker of kidney function. It is inaccurate at detecting mild renal impairment, and levels can vary with muscle mass and protein intake.[4] Formulas such as the Cockcroft and Gault formula and the MDRD formula (see Renal function) try to adjust for these variables.
Cystatin C has a low molecular weight (approximately 13.3 kilodaltons), and it is removed from the bloodstream by glomerular filtration in the kidneys. If kidney function and glomerular filtration rate decline, the blood levels of cystatin C rise. Serum levels of cystatin C are a more precise test of kidney function (as represented by the glomerular filtration rate, GFR) than serum creatinine levels.[3][5] This finding is based mainly on cross-sectional studies (on a single point in time). Longitudinal studies (that follow cystatin C over time) are scarcer; some studies show promising results.[6][7][8] Cystatin C levels are less dependent on age, sex, race and muscle mass compared to creatinine. Cystatin C measurements alone have not been shown to be superior to formula-adjusted estimations of kidney function.[9] As opposed to previous claims, cystatin C has been found to be influenced by body composition.[10][11] It has been suggested that cystatin C might predict the risk of developing chronic kidney disease, thereby signaling a state of 'preclinical' kidney dysfunction.[12]
Studies have also investigated cystatin C as a marker of kidney function in the adjustment of medication dosages.[13][14]
Cystatin C levels have been reported to be altered in patients with cancer,[15][16][17] (even subtle) thyroid dysfunction[18][19][20] and glucocorticoid therapy in some[21][22] but not all[23] situations. Other reports have found that levels are influenced by cigarette smoking and levels of C-reactive protein.[24] Levels seem to be increased in HIV infection, which might or might not reflect actual renal dysfunction.[25][26][27] The role of cystatin C to monitor GFR during pregnancy remains controversial.[28][29] Like creatinine, the elimination of cystatin C via routes other than the kidney increase with worsening GFR.[30]
I didn't consider the 24 hour urine, although that may be difficult in patients like Izz that can't hold their bowels while urinating. (We tried using two hats at the hospital-better than nothing but still didn't work particularly well lol!)
