Therapeutic Drug Monitoring PDF

4 I LAB VA LUE S & DR UG M ON ITO RIN G THERAPEUTIC DRUG MONITORING Drug levels or other values (such as anti-Xa levels for LMWHs) are used to reach dosing goals and avoid toxicity. Therapeutic drug monitoring (TDM) is increasingly common due to the need to target highly resistant organisms and dos...

4 I LAB VA LUE S & DR UG M ON ITO RIN G THERAPEUTIC DRUG MONITORING Drug levels or other values (such as anti-Xa levels for LMWHs) are used to reach dosing goals and avoid toxicity. Therapeutic drug monitoring (TDM) is increasingly common due to the need to target highly resistant organisms and dose medications properly in overweight and obese patients. The peak level is the highest concentration in the blood the drug will reach and requires time for the drug to distribute in the body's tissues. The trough level is the lowest concentration the drug will reach in the blood and is dt·awn right before the next dose or some short period of time before the next dose (30 minutes is common). This allows time to assess the level before another dose is given and time to hold the next dose if the level is high. The time that drug levels are drawn is critical for accurate interpretation. For example, a tobramycin level of 6 mcg/mL would be interpreted differently if the level was a trough versus a peak. Obtaining drug levels at steady state is often (but not always) preferred. See Pharmacokinetics chapter for further discussion. Narrow therapeutic index (NTI) drugs have a narrow separation between the subtherapeutic (low), therapeutic (desired) and supratherapeutic (high) drug levels. Supratherapeutic drug levels can be toxic. TDM is commonly performed by pharmacists. The following Key Drugs Guy lists drugs that are routinely monitored. These drugs and usual therapeutic ranges are felt to be essential fo1• NAPLEX. THERAPEUTIC DRUG LEVELS DRUG USUAL THERAPEUTIC RANGE Carbamazepine 4-12 mcg/ mL l 0.8-2 ng/ mL (AFib) lJigoxin 0.5-0.9 ng/ mL (HF) Peak: 5- 10 mcg/mL Gentamicin (traditional dosing) I Trough: < 2 mcg/ mL j 0.6-1.2 mEq/L (up to 1.5 mEq/ L fo~acute symptoms), drawn a; a trough Lithium Phenytoin / Fosphenytoin 10-20 mcg/ mL; if albumin is low, correct serum level; see Seizures/ Epilepsy chapter Free Phenytoin 1-2.5 mcg/mL 1 4-10 mcg/mL NAPA (procainamide active 15-25 mcg/ mL Procainamide - ---------- ---------------- metabolite) Combined - Theophylline Tobramycin (traditional dosing) 10-30 mcg/ mL 7 -- 5-15 mcg/mL Peak: 5- 10 mcg/ mL Trough:< 2 mcg/mL Valproic acid 50-100 mcg/mL (up to 150 mcg/mL in some patients); if albumin is low, correct serum level; see Seizures/Epilepsy chapter Vancomycin* Trough: 15-20 mcg/mL for most serious infections (pneumonia, endocarditis, osteomyelitis, meningitis, and bacteremia) Trough: 10- 15 mcg/ml for others Warfarin Goal INR is 2-3 for most indications, use higher range (2.5-3.5) for high-risk indications, such as mechanical mitral valves 'AUC can be used to monitor vancomycin. Refer to the Pharmacokinetics chapter for detailed information. Select Guidelines/References Lab Tests Online. https://labtestsonline.org (accessed 2021 M ar 17). Lee M . Basic Skills in Interpreting Laboratory Data. 6 th ed. Betheseda, MD: ASHP; 2017. Schmidt J, Wieczorkiewicz J. Interpreting Laboratory Data: A Point-of-Care Guide. Betheseda, MD: ASHP; 2012. 82