Rx Calc

Introduction

When I was a provisional registered pharmacist in TDM rotation, I found that pharmacokinetics calculation is a repetitive task and sometimes, the calculation results are just an aid in dosage adjustment. Hence, I decided to create TDM calculator using Microsoft Excel.

The initial design of the TDM calculator spreadsheet is



Excel Sheet

The final Excel sheet (last updated 1 Jan 2022) can be opened using Microsoft Office for both desktop and mobile phones.

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

If you would like to verify the equations or make some self-modifications, you may remove the protection from editing.

  • Go to Review tab, click Unprotect Sheet and Unprotect Workbook. *No password is set*
  • Click "control + A" or select all, change the text colour to black to unhidden the cells.
  • After modification, remember to click Protect Sheet to avoid any accidental changes.

NOTE: You may read this post to learn the excel skills used.



Open As App

Alternatively, you may also access the same spreadsheet through Open As App for better mobile interactive experience.



Read These Before Use

As expected, what application do is to produce results based on your input. Hence, to maximize the use of these calculators, you have to considers the basics of TDM.

  • First, you must set the setting options at the top.
  • Yellow boxes are blank that can be filled, while white boxes are calculation result based on the input data. However, not all yellow boxes must be filled. You can be selective in data input.
  • The patient current dose and obtained concentration is needed to calculate individual pharmacokinetics. However, if the level is not taken at steady state or the dose is just a loading dose (or stat dose), these data should not be keyed in, just leave them blank. When individual data is not provided, the estimation will proceed using population data.
  • There is many a population pharmacokinetic equation available. Users should decide the equations based on scenarios and workplace practice.
  • All values produced are just still an estimation, you still need to treat the patient first, not the data.



Possible Causes of Variation

The Adjusted Body Weight is calculated using formula below.

Adj Body Weight = Ideal Body Weight + 0.4 * (Actual Body Weight - Ideal Body Weight)

The 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 the population parameter is in a range value, the midpoint of the values is used for estimation.

In the Excel spreadsheet for phenytoin, it is suggested to calculate corrected phenytoin level when albumin is < 25 g/L. However, this decision can vary across facilities, hence you can still choose corrected phenytoin level to proceed with further calculation if needed.

Slight variation in result could be attributed to rounding of the values calculated.



Final Reminder

Before you start using the calculator in your daily work, it is a must to test run it along with few cases that you have calculated manually. Without a clear understanding on the information needed for each calculation, this will make the TDM calculator to be of obselete value.

Bear in mind that this is a self-initiative project and I do not guarantee its accuracy.

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