Therapeutic Drug Monitoring (TDM)
TDM is useful in following situations
- The drug in question has a narrow therapeutic range
- The therapeutic effect cannot be easily assessed by clinical observation
- Poor relationship between dose and serum drug concentration
- Non-linear pharmacokinetics
- Difficulty in interpreting signs and symptoms of toxicity or therapeutic failure or therapeutic responses
- Dosage adjustment can not be made by clinical parameters which may lead to serious toxicity.
- Is utilized to monitor patient's compliance with the prescribed drug regime
High Performance Liquid Chromatography (HPLC) in TDM HPLC is considered as the 'Gold Standard' among analytical techniques. It is used for accurate estimation of drugs useful both for Therapeutic Drug Monitoring (TDM) and drug overdose. With HPLC, the parent drug and its metabolites (both active and inactive) can be measured separately. Whereas estimation by immunoassay can give erroneous results (e.g. methotrexate). Determination of pharmacologically active metabolite by HPLC presents an added advantage over immunoassay in TDM. The examples of such drugs with active metabolites include: Carbamazepine & its Epoxide (ii) Oxcarbazepine & its mono-Hydroxy metabolite.
| Blood Paracetamol and Salicylates by spectrophotometry |
Blood levels of certain common drugs (anagesics) like paracetamol and salicylates are useful in TDM.
Blood levels of paracetamol are estimated after four hours of overdose to decide about the need for N-acetyl cysteine (as per Rumack-Mathew Nomogram). |
| Antiepileptic Drugs by HPLC |
-Phenobarbital, Phenytoin, Carbamazepine & Its Epoxide, Oxcarbazepine & its mono-Hydroxy metabolite
-Ethosuximide, Lamotrigine, Primidone, Sulthiame -Lacosamide, Zonisamide, Felbamate, Rufinamide, N-Desmethylmethsuximide Levetiracetam |
| Antifungal drugs by HPLC | Blood levels of Voriconazole, Posaconazole |
| Antibiotics by HPLC | Blood levels of Vancomycin |
| Anticancer drug | Methotrexate from blood |
Guideline for sample collection-TDM Tests |
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| Drug | Test | Sample Requirement | TAT |
| Antifungal drugs | Blood Voriconazole Level | 1ml Separated Plasma/Serum | 6 hours |
| Blood Posaconazole Level | 1ml Separated Plasma/Serum | 6 hours | |
| Antiepileptic Drugs-I | Phenobarbitone, Phenytoin, Carbamazepine & its epoxide, Oxcarbazepine & its mono-Hydroxy metabolite | 1 ml Separated Serum | 5 hours |
| Other Antiepileptic Drugs-II | Ethosuximide, Carbamazepine, Carbamazepine-epoxide, Ethosuximide, 10-Hydroxy-Carbamazepine, Lamotrigine, Oxcarbazepine, Phenobarbital, Phenytoin, Primidone, Sulthiame | 1 ml Separated Serum | 6 hours |
| Other Antiepileptic Drugs-III | Lacosamide, Zonisamide, Felbamate, Rufinamide, N-Desmethylmethsuximide | 1ml Separated Plasma/Serum | 6 hours |
| Other Antiepileptic Drugs-IV | Blood Levetiracetam | 1ml Separated Plasma/Serum | 6 hours |
| Analgesics |
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3 ml EDTA Blood /1 ml serum Blood collection at least 4 hours after ingestion; the time of 'drug ingestion' and 'blood collection' to be mentioned | 2 hours |
| NB: (i) Blood/Serum must not be hemolyzed and Gel tubes not be used which cause interference in HPLC analysis. Blood should be collected; serum/plasma separated as soon as possible and stored properly or transported at 2 -8℃ (ii) For trough Level of the drug, collect the sample before the next dose. | |||