Low serum creatinine is caused by conditions that affect the muscles, including those that affect only the muscles and those that also affect the nerves that connect to the muscles, according to MedlinePlus. Specific conditions that affect only muscle include muscular dystrophy, while conditions including myasthenia gravis also involve nerves.
The usual reason for a decrease in creatinine levels is that the muscles are using less fuel, MedlinePlus says. This can be because the muscles have wasted away, as occurs in advanced muscular dystrophy, or it can occur because the muscles are abnormally underutilized, as occurs with myasthenia gravis. Myasthenia gravis is an autoimmune disease where the body's immune system attacks the neurotransmitters that nerves use to activate muscles. It causes severe muscle weakness and can cause muscles, including breathing muscles, to slacken and fail to activate when needed.
Creatinine is a waste product from processing creatine, a substance made by the body to provide fuel for muscles, according to MedlinePlus. The tests are commonly performed to check for kidney function, where a high blood level of creatinine indicated impairment because it is removed from the blood only by the kidneys. Creatinine levels also vary depending on a person's size and muscle mass. Women typically have a lower level of creatinine than men.