Creatinine is a substance produced in the muscles by metabolism. It’s a waste product filtered from the blood by the kidneys. When creatinine levels in the blood are tested as lower than normal, this is (in part) a sign that the kidneys are functioning well, or at least, unlike elevated creatinine, not a sign to the contrary. Elevated creatinine is a reason for concern. Low creatinine levels normally are not, at least not without other diagnostic indications. All of this applies to serum (blood) creatinine levels taken in isolation. Of much greater importance is the ratio of serum creatinine to creatinine in urine.

Causes 

Some of the causes of low creatinine levels in the blood include a low-protein diet, pregnancy, old age, decreased muscle mass, and liver disease. Neither of the first two is a cause for concern unless the protein intake is low enough to threaten health or there are complications to pregnancy (of which low creatinine level is not a particular indication). Old age can certainly bring health problems, but low creatinine level
is not a reliable diagnostic of geriatric difficulties. Loss of muscle mass and liver disease, of course, represent health problems, but in both cases other indications will be found and a low creatinine level by itself is not cause for concern.

Low Creatinine Levels In Urine Tests

While low creatinine levels in the blood are actually a good sign most of the time, low creatinine levels in urine can be a bad one. The kidneys filter this waste product from the blood and put it into the urine, so we should expect urine creatinine levels to be higher than blood creatinine levels.

A low creatinine level in urinalysis combined with elevated creatinine in the blood shows at least partial renal dysfunction which can be a serious condition. However, low urine creatinine levels by themselves can simply show that low levels of creatinine are being produced by the metabolism for any of the reasons mentioned above.

The important indicator is elevated creatinine in the blood, rather than low creatinine in the urine; however, the two together do help to pinpoint the problem as arising in the kidneys rather than elsewhere.

In fact, the important consideration is not either test in isolation so much as the relationship between the two. When creatinine levels in the blood are higher than in the urine, that is an indication of a disorder of the kidney and requires further investigation with additional diagnostic procedures to determine the source and nature of the disorder.

Women

Women on the average have lower muscle mass than men, and therefore produce less creatinine. Creatinine levels in the blood are normally lower for women than for men. What this means in practice is that a lower baseline level of blood creatinine is used for women than for men.

A serum creatinine level that would be considered “low” for a man is considered normal in a woman. In addition, those of African descent have higher normal serum creatinine levels than those of non-African ancestry, hence a slightly higher baseline for Africans is used, whether male or female.

Children

Children also have lower muscle-mass than adults, and so serum creatinine levels are normally lower in children than in adults. As with women, a different baseline is used based on the age of the child and the body weight or estimated muscle mass. As in all cases, elevated serum creatinine is of greater concern than low creatinine levels.