Creatinine is a waste product of normal muscle metabolism. It is easily filtered out of the blood by well-functioning kidneys, and levels typically remain relatively stable in the body and bloodstream over time. Creatinine is often included in routine laboratory testing to evaluate kidney function. There are many reasons why creatinine levels may be low, some as relatively harmless as a low-protein diet and others as serious as advanced liver disease. A low creatinine level alone often does not indicate a serious medical condition and can be caused by multiple factors.
Low Muscle Mass
When the kidneys are functioning normally, a creatinine level most closely reflects the amount of muscle in the body. Low muscle mass is a leading cause of a low creatinine level. For example, as people age, their creatinine level may decrease as they lose muscle mass -- particularly if they are not exercising regularly. Another cause of low creatinine is severe malnutrition that leads to muscle loss. When the intake or absorption of calories from food is severely reduced, the body needs an alternate energy source to make up for the insufficient calories. Fat and protein breakdown -- primarily from muscle tissue -- provides needed energy with low calorie intake or absorption. Prolonged starvation or illness that reduces nutrient absorption can lead to significant loss of muscle mass and a low creatinine level.
Diet can also impact creatinine level. A vegetarian diet is a relatively common cause of a lower-than-expected blood creatinine. Animal protein from meat, poultry and fish contributes to the blood creatinine level because they contain large amounts of creatine, which is then metabolized to creatinine. Plant-based foods do not directly affect the blood creatinine level as they contain no creatine. Since vegetarians do not eat animal protein, their creatinine levels tend to be lower than those of nonvegetarians.
Pregnant women tend to have a reduced blood creatinine level. This occurs, in part, because the kidneys more efficiently clear creatinine and other waste products from the blood during pregnancy. Expansion of the liquid portion of the blood during pregnancy also contributes to a lower creatinine level due to a dilution effect. Creatinine levels typically return to expected levels after delivery.
Less Common Causes
Advanced liver disease can lead to a low creatinine level. The liver is a primary site for protein manufacture and breakdown in the body. When the liver is not functioning well, proteins are not made or broken down efficiently, potentially causing a low creatinine level. Severe, excess water intake is an uncommon cause of low blood creatinine. In this circumstance, an overabundance of water in the circulation dilutes the concentration of all substances in the bloodstream. Some medications may interfere with certain methods used to measure blood creatinine, potentially causing an erroneous reading.
Determining Cause and Significance
A low creatinine level alone does not necessarily mean that there is an underlying health problem. There are a number of reasons a creatinine level may be lower -- or higher -- than the reference range noted by the testing laboratory. The doctor will look at many factors in deciding whether there is cause for concern. Important considerations include current medications, diet, ongoing illnesses and whether a women is pregnant. Additional laboratory testing or other examinations may be needed to determine whether a low creatinine level represents a health concern.
Reviewed by: Tina M. St. John, M.D.