Also known as: Creat. Serum creatinine level is proportional to lean body muscle mass, unaffected by most diet or activity, and is freely filtered by the glomerulus. Both creatinine and BUN are often ordered to follow renal problems. Creatinine overall is the more reliable index, but each has pitfalls. As creatinine increases in chronic renal failure, the hematocrit decreases, total carbon dioxide and bicarbonate fall, and BUN and serum phosphate increase. When serum creatinine increases postoperatively, patients may be identified who are at risk for more severe renal failure. Creatinine clearances are useful in such investigations.