Measuring serum creatinine

A normal plasma creatinine level does not necessarily mean that renal function is normal. In this video, from our Kidney Function Test Essentials course, we'll take a look at the physiology of creatinine, what we need to consider when interpreting creatinine levels, and why GFR is not always an accurate reflection of kidney function.

Master the ins and outs of kidney function tests! Order and interpret these tests with complete confidence, knowing that you’ve conquered this critical skill. Taking a shotgun approach to kidney function testing can be tempting if nephrology isn't your forte. By the end of this course, however, you’ll be comfortable ordering and interpreting all of the common kidney function tests. You’ll understand the clinical implications of abnormalities seen on these tests and be able to form a practical plan of action.

Creatine is synthesized in the kidneys, liver and pancreas. It is transported in the blood to other organs, especially brain and muscle, where it is phosphorylated to phosphocreatine. Phosphocreatine is a high energy compound, an interconversion of phosphocreatine to creatine is important to muscle function. Creatinine is the waste product derived from creatine and phosphocreatine.

Creatine in production is related to the muscle mass of an individual. The higher the muscle mass, the greater the creatine in production. Women usually excrete 1.2 grams of creatinine per day, while men excrete 1.5 grams per day. Serum creatinine levels are affected by gender, age, weight, lean body mass and dietary protein intake.

Creatinine is filtered and excreted by the kidney, so plasma creatinine is inversely related to the glomerular filtration rate, or GFR. However, GFR can decrease by up to 50 percent before plasma creatinine concentration rises above the normal range. So, what should we consider when interpreting the plasma creatinine level?

A normal plasma creatinine level does not necessarily mean renal function is normal. And changes in creatinine levels can occur without changing renal function, for example, due to a change in muscle mass. Also note that in pregnancy, due to an increasing GFR, the level of creatinine typically falls.