Volume 7 Issue 3 March, 2017
Consultation Liaison Psychiatry Focus: Biochemistry
Drugs used in Psychiatric practice frequently have narrow therapeutic indices and their serum levels need to be monitored for patients’ safety and desired drug action.
The tricyclic anti-depressants have almost complete GI absorption but they have a significant first pass metabolism at the liver. The drugs reach a peak plasma concentration within 2 to 12 hours following an oral dose and show a good correlation between the serum levels with therapeutic response. The drugs are metabolized majorly by the CyP2C19 and 2D6 systems. Chromatography of serum (Gas Chromatography and Mass spectrometric analysis) for the serum levels of the drugs or their metabolites is considered the gold standard for monitoring them.
Lithium, administered as Lithium Carbonate is postulated to enhance the reuptake of catecholamines, thereby reducing their availability at the neuronal junctions. This has a sedative effect on the central nervous system. Lithium absorption is complete and reaches a peak plasma concentration within 2 to 4 hours after an oral intake. Lithium does not bind to plasma proteins and its excretion from the body is bi-phasic. During the first phase, 20-30% of the Lithium is cleared with an apparent half-life of 24 hours. During the second phase, the remainder of the Lithium incorporated in the cellular ion pool is cleared, with a half-life of 48 to 72 Hrs. Clearance is a function of the kidneys, where active re-absorption of the ion also occur. Thus reduced renal clearance may prolong clearance of the ion. Toxicity is directly related to the serum levels and therefore serum concentrations are monitored to monitor drug compliance and to avoid intoxication. The venous blood sample is sent to the lab in a plain glass or plastic vial. Since even a clot activator vial is found to interfere with estimation of this ion, they are best avoided. The sample is allowed to clot at room temperature following which it is centrifuged to settle all formed elements of the blood and get a clear serum. Lithium is estimated in the serum using an Ion Selective Electrode. This method is found to be much better than the colorimetric or emission spectrophotometric methods. If the Lithium estimation is planned for later, the serum may be separately stored in small, labelled, air-tight plastic micro centrifuge tubes and stored at minus twenty freezers. Some workers have shown a very high correlation between the lithium levels in serum and saliva, pointing to the possibility that saliva may be used as a non-invasive substitute for serum for lithium monitoring.
Department of Biochemistry, AIIMS, Bhubaneswar
