Acetaldehyde is so highly reactive that it is difficult to trace, especially in the body. However, in the presence of thiamine-pyrophosphate, acetyl-coenzyme A and NADH it reacts to form a stable metabolite called 2,3-butanediol. Note that the resulting molecule has the structure of two acetaldehyde molecules butted end to end. One of the acetyl groups comes from acetyl-coenzyme A and the other from free acetaldehyde.
2,3-butanediol has been found in the urine of humans in "apparently unrelated disease states".