RxCalc

Introduction

During my time as a provisionally registered pharmacist rotating through Therapeutic Drug Monitoring (TDM), I realized that pharmacokinetic calculations were highly repetitive tasks.

• While pharmacokinetic modeling is useful, clinical decisions to withhold or adjust narrow therapeutic index drugs ultimately rely on the patient's clinical response. For instance, we monitor CRP, temperature, and white blood cell count for vancomycin and gentamicin, while actively tracking creatinine clearance changes for signs of toxicity.
• Mathematically, simply looking at a measured drug level often allows us to easily decide whether to withhold, adjust, or continue the current dose. This frequently renders subsequent pharmacokinetic calculations redundant.
• Consequently, I concluded that these calculations are primarily beneficial for dosage adjustments in toxicity cases.

To streamline this process, I created an Excel-based TDM calculator in Excel sheet and OpenAsApp Link. The spreadsheet was designed with the following structure:

• One dedicated sheet per drug to maintain clarity.
• Adaptability to accommodate diverse clinical scenarios.
• Flexibility allowing users to select their preferred population pharmacokinetic equations.
• Evidence-based design using Clinical Pharmacokinetics Pharmacy Handbook, 2019 as the main reference.

Excel Sheet User Guide

The final Excel sheet (last updated 1 Jan 2022) is fully compatible with Microsoft Office on both desktop and mobile devices. It includes modules for:

• Patient Profile
• Aminoglycosides
• Carbamazepine
• Digoxin
• Phenobarbitone
• Phenytoin
• Sodium valproate
• Vancomycin

If you wish to verify the equations or customize the sheet, you can easily remove the editing protections:

• Go to Review tab, then click Unprotect Sheet and Unprotect Workbook (no password is required).
• Press "control + A" (or select all cells) and change the text colour to black to reveal hidden calculation cells.
• After making your modification, remember to click Protect Sheet to prevent accidental formula overwrites.

As with any application, the accuracy of the output depends heavily on the quality of your input data. To maximize the utility of these calculators, you must consider the core principles of TDM.

• First, configure the setting options at the top of the sheet.
• Yellow boxes represent blank fields that can be filled, whereas white boxes display calculation results generated from your inputs. You do not need to fill every yellow box; data input can be selective.
• The patient’s current dose and measured concentration are required to calculate individual pharmacokinetics. However, if the blood level was not taken at steady state, or if the dose administered was merely a loading (or stat) dose, leave these fields blank. When individual data is omitted, the tool will automatically fall back on population-based estimates.
• Because numerous population pharmacokinetic equations exist, users should select the formula that best fits their specific clinical scenario and institutional workplace practice.

Remember: All generated values are merely estimations. Always treat the patient first, not the data.

Calculations & Sources of Variation

The spreadsheet utilizes specific mathematical formulas to standardize patient data:

• The Adjusted Body Weight is calculated using formula below.

Adj BW = Ideal BW + 0.4 * (Actual BW - Ideal BW)

• For pediatric patients, CrCl calculated using Schwartz Formula has been multiplied with [ Patient's body surface area (BSA) / 1.73 ]. The BSA formula used is Mosteller Formula.

BSA (m²) = [ height (cm) * weight (kg) / 3600 ]^0.5

• When a population parameter is provided as a range, the midpoint value is used for estimation.
• Minor discrepancies in final results may occur due to the rounding of intermediate values.

Summary

Years later, I took the initiative to leverage "vibe coding" to build a more modernly designed TDM calculator Progressive Web App (PWA) called: RxCalc.

• Moving from a rigid spreadsheet layout to a web app offered far more flexibility in terms of user interface and workflow design.
• However, the foundational rules of TDM remain unchanged: simply entering data into designated fields without clinical context will only yield values of obsolete utility.

Practitioners must remain well aware that a patient's individual pharmacokinetic parameters always supersede the population PK parameters typically used in empirical estimations.

• Furthermore, whenever a measured drug level is highly supratherapeutic, clinicians should know how to use the estimated half-life from pharmacokinetic calculations to determine the exact time needed to withhold therapy and schedule subsequent resampling.

Lastly, please bear in mind that this is an independent, self-initiated project, and I do not guarantee its absolute accuracy in clinical settings.