Blood drug concentration is measured by laboratory tests and cannot be derived from a formula. Blood drug concentration refers to the concentration of a drug in a patient’s blood over a certain period of time after taking the drug. There are three methods for measuring blood drug concentration: spectroscopy, chromatography, and immunoassay, with chromatography being the most commonly used method. Generally, the concentration of a drug in the plasma is directly proportional to the efficacy of the drug, and the therapeutic effect of the drug can be better maintained if the blood concentration remains stable after the drug is used. If the blood concentration fluctuates too much, it is not conducive to the smooth control of the disease. Only good monitoring can better use of drugs, accurately determine the drug taking program, to avoid the recurrence of the disease or drug overdose. For those drugs whose effective concentration is close to the toxic blood concentration, as well as patients who have been using drugs for a long time or using drugs in combination, it is especially important to pay attention to the monitoring of drug concentration to prevent the drug concentration in the body from being too high and producing toxic side effects.