Therapeutic Drug Monitoring Calculator

Therapeutic Drug Monitoring

Clinical Calculator · Pharmacy Department, Sibu Hospital

Ideal Body Weight & Adjusted Body Weight

Height (cm)

Actual Body Weight (kg)

IBW (Devine) = 50 kg (male) / 45 kg (female) + 2.3 kg per inch over 5 ft. Adjusted BW = IBW + 0.4 × (actual − IBW), shown only when actual weight exceeds IBW.

Creatinine Clearance Cockcroft–Gault

Weight (kg)

Age (years)

Serum Creatinine (µmol/L)

CrCl (mL/min) = (140 − age) × weight × 1.23 (male) or 1.04 (female) ÷ serum creatinine (µmol/L).

Rumack–Matthew Nomogram Toxicity

Measured concentration (mcg/mL)

Hours post ingestion

Treatment line = 300 × 0.84^h mcg/mL (equivalent to 150 mcg/mL at 4 h). High-risk line = 200 × 0.84^h mcg/mL. Valid for a single acute ingestion, 4–24 hours post ingestion.

One Steady State Concentration Adjust dose

Estimate the dose required to reach a targeted concentration.

Current dose (mg/day)

Measured concentration (mcg/mL)

Targeted concentration (mcg/mL)

Predict Level From Current Dose

Current dose (mg/day)

Measured concentration (mcg/mL)

Suggested dose (mg/day)

Two Steady State Concentrations Ludden

Dose 1 (mg/day) — higher

Concentration 1 (mcg/mL)

Dose 2 (mg/day) — lower

Concentration 2 (mcg/mL)

Targeted concentration (mcg/mL)

Km = (D₁ − D₂) ÷ (D₁/C₁ − D₂/C₂) · Vmax = D₁ × (Km + C₁) ÷ C₁ · D_new = Vmax × C_target ÷ (Km + C_target).

Corrected Phenytoin Concentration

Measured concentration (mcg/mL)

Albumin (g/L)

Hypoalbuminaemia: C_corr = C ÷ (0.2 × albumin/10 + 0.1). Renal failure: C_corr = C ÷ (0.1 × albumin/10 + 0.1).

Incremental Loading Dose

Body weight (kg)

Measured concentration (mcg/mL)

Desired concentration (mcg/mL)

Loading dose = 0.7 L/kg × body weight × (desired − measured) ÷ 0.92.

Time To Withhold Toxic

Measured concentration (mcg/mL)

Desired concentration (mcg/mL)

Days to withhold = [4 × ln(C_measured/C_desired) + (C_measured − C_desired)] × 0.7 ÷ 7.

Patient Specific Pharmacokinetic Data Two levels

For drug infused over 30–60 mins. Not applicable if the drug was infused over more than 1 hour.

Body weight (kg)

Dose (mg)

Dosing interval (hour)

Concentration 1 (mcg/mL)

Post (hour)

Concentration 2 (mcg/mL)

Post (hour)

Ke = ln(C₁/C₂) ÷ (t₂ − t₁) · t½ = 0.693/Ke · Cmax = C₁ × e^Ke(t₁−1) · Vd = dose ÷ (Cmax − Cmin).

Suggested New Dose To adjust dose

Target Cmax (mcg/mL)

Dosing interval (hour)

Uses the Vd derived above. Dose_new = Vd × (Cmax_target − Cmin_estimated).

Time Ideal & DFP To adjust interval

DFP = ln(Cmax/Cmin) ÷ Ke + infusion time. Time ideal uses a 10 : 1 peak-to-trough ratio.

Patient Specific Pharmacokinetic Data

Dose (mg)

Cpre (mcg/mL)

Pre (hour)

Cpost (mcg/mL)

Post (hour)

Dosing interval (hour)

Infusion time (hour)

Ke = ln(Cpost/Cpre) ÷ [interval − (pre + post)] · Cmax = Cpost × e^{Ke(post − infusion)} · Cmin = Cmax × e^{−Ke(interval − infusion)} · Vd = dose ÷ (Cmax − Cmin).

Estimating New Dose Patient specific PK

Target Cmax (mcg/mL)

Suggested dosing interval (hour)

Estimating Dose Population PK

Age (year)

Actual body weight (kg)

Serum creatinine (µmol/L)

Dose (mg)

Dosing interval (hour)

Infusion time (hour)

Ke = [0.695 × (CrCl/ABW) + 0.05] × 60 ÷ 700 h⁻¹ · Vd = 0.7 L/kg · Cmax = (dose ÷ Vd·BW) ÷ (1 − e^−Ke·τ) · AUC₂₄ = daily dose ÷ (Ke × Vd).

Targets & Suggested Regimen

Target Cmax (mcg/mL)

Target Cmin (mcg/mL)

Loading dose = Vd × BW × Cmax_target. Interval = ln(Cmax/Cmin) ÷ Ke + infusion time.

Patient Specific Pharmacokinetics From levels

Cpre (mcg/mL)

Pre (hour)

Cpost (mcg/mL)

Post (hour)

Dosing interval (hour)

Infusion time (hour)

Body weight (kg)

Dose (mg)

Cpss Antiepileptics Steady state

Total daily dose (mg)

Body weight (kg)

Phenytoin Cpss uses the non-linear Michaelis–Menten equation (Km 4 mg/L, Vmax 7 mg/kg/day, F·S 0.92): Cpss = 4 × 0.92 × dose ÷ (7 × weight − 0.92 × dose). Carbamazepine and valproic acid rows reproduce the clearance coefficients of the original application; polytherapy clearance is higher, so the predicted level is lower.

Therapeutic Drug Monitoring Serum Sampling Guide

Blood sample needed for analysis — adult 3 mL, paediatric 2 mL. Use plain tubes for all drugs except cyclosporin, tacrolimus and sirolimus (EDTA tube).

About

Therapeutic Drug Monitoring Calculator

This application is developed by HCKS and TSC
Pharmacy Department, Sibu Hospital

Disclaimer. This application is not a substitute for clinical judgement and professional medical advice. The developers expressly disclaim responsibility, and shall have no liability over any errors incurred as a result of your reliance on the information obtained from this application.

References

Ministry of Health Malaysia, 2019. Clinical Pharmacokinetics Pharmacy Handbook, 2nd Edition.
White, S. and Rumack, B., 2005. The Acetaminophen Toxicity Equations: "Solutions" for Acetaminophen Toxicity Based on the Rumack–Matthew Nomogram. Annals of Emergency Medicine, 45(5), pp.563-564.
Rozario, F.D, Soo, H.H, Hu, M., Chew, P.H, 2020. Sarawak Handbook of Medical Emergencies, 4th Ed. Malaysia: CE Publishing.
Bauer, L., 2008. Applied clinical pharmacokinetics. 2nd ed. United States: McGraw Hill.
Borang Permohonan Therapeutic Drug Monitoring Versi 1 2022.

Web edition

This web version was rebuilt from the original Android application. Calculations, section structure and therapeutic ranges follow the original app. Layout and typography were redesigned for the browser.