Creatinine is a protein that is created as a waste byproduct of energy metabolism in muscle. As it is created, creatinine is transported to the kidneys and filtered into urine for excretion. The levels of this protein in the blood are a reliable measure of how well the kidneys function, because normal creatinine levels in the blood remain consistent from day to day. Normal creatinine levels range between 0.5 to 1.2 milligrams per deciliter (one-tenth of a liter) of blood, depending on the sex of the individual.
Normal creatinine levels are different for different people, because what constitutes normal for each individual depends on several factors, including sex and age. Although there is variation on an individual basis, in any given person, creatinine levels tend to stay the same on a day-to-date basis. In the long term, creatinine levels fluctuate for several reasons.
This fluctuation creatinine levels is because normal levels largely depend on a person’s muscle mass. Therefore, as long as muscle mass remains constant, the amount of creatinine produced also remains constant. Men tend to have higher creatinine levels than women, and creatinine levels drop as a person ages. This correlates with muscle mass, which tends to be higher in men than in women and reduces with age in both sexes.
The consistency of creatinine levels in blood and urine assumes that the kidneys remain healthy. This is because the kidneys filter creatinine from the blood, and it is excreted in urine. If kidney function is impaired, these organs are not able to maintain a normal rate of creatinine clearance. As a result of impaired kidney function, blood levels of creatinine are higher than normal, because the protein is not being removed from the blood at a normal rate. Urine levels can be higher or lower than normal, depending on the type of kidney impairment involved.
There are many diseases that can cause a deviation from normal creatinine levels, including kidney diseases and others not directly related to kidney function. In adults, the most common causes of elevated blood creatinine levels are diabetes and high blood pressure. Other kidney diseases, including inflammatory conditions such as glomerulonephritis and pyelonephritis, can cause elevated blood creatinine. Conditions that are not related to the kidneys but can cause elevated creatinine include urinary tract obstruction and an autoimmune disease called myasthenia gravis.
In addition, there are some reasons why blood creatinine can be higher than normal even when the kidneys are healthy. Some drugs can increase blood creatinine artificially, and a diet that is high in meat also can cause elevated blood creatinine levels. In addition, someone who is a bodybuilder or another muscular type of athlete is likely to have high creatinine levels in the blood. This is because a bodybuilder or muscular athlete has much higher muscle mass than an average person of the same sex, size and age.