Almost all organic acids used for human testing are measured by a combination of gas or liquid chromatography linked with mass spectrometry. Organic acids are most commonly analyzed in urine because they are not extensively reabsorbed in the kidney tubules after glomerular filtration. Thus, organic acids in urine are often present at 100 times their concentration in the blood serum and thus are more readily detected in urine. This is why organic acids are rarely tested in blood or serum. The number of organic acids found in urine is enormous. Over 1,000 different organic acids have been detected in urine since this kind of testing started.