What does a high level of lactic acid mean?

According to WebMD, high levels of lactic acid mean that the body's lactate threshold, also known as the anaerobic threshold, has been passed, and that lactic acid is building up in the kidneys and liver. While lactic acidosis typically causes fatigue and muscle aches, it can be potentially life-threatening.

WebMD explains that lactic acid levels generally become high due to strenuous exercise but can also increase due to heart failure, infection, shock or as a side effect of metformin, a drug taken to control diabetes. Symptoms of severe lactic acidosis include rapid breathing, belly pain, confusion, clammy skin, nausea, sweating, sweet-smelling breath and coma.