About This Calculator

The Polymyxin B Dose Calculator is designed to assist healthcare professionals in determining appropriate loading and maintenance doses for intravenous (IV) polymyxin B therapy in adult patients. It utilizes the Cockcroft-Gault equation to estimate renal function and applies current evidence-based dosing principles for this last-line antibiotic.

Outputs Explained

Upon entering the required patient data, the calculator provides the following key outputs for a personalized dosing schedule:

• Recommended Loading Dose: The initial, larger dose calculated to rapidly achieve therapeutic drug concentrations. Provided as a range in milligrams (mg) or International Units (IU).
• Recommended Maintenance Dose: The subsequent doses required to maintain therapeutic concentrations. Provided as a range (mg or IU) with a recommended frequency of every 12 hours.
• Estimated Creatinine Clearance (CrCl): An estimate of the patient’s renal function in mL/min, calculated using the Cockcroft-Gault formula, which helps inform clinical assessment.

How to Use the Tool

To ensure an accurate dose calculation, please provide the following patient parameters:

• Patient Weight: Use the patient’s Actual Body Weight (ABW). This is critical as all calculations are based on ABW.
• Patient Height: Used in conjunction with other parameters for a complete patient profile.
• Serum Creatinine (SCr): A key marker of kidney function.
• Patient Age: An essential variable in the Cockcroft-Gault equation.
• Patient Sex: The Cockcroft-Gault formula includes a correction factor for female patients.

Dosing Overview

Polymyxin B dosing is based on a patient’s actual body weight to ensure efficacy while minimizing toxicity. A typical regimen involves:

• Loading Dose: A single dose of 2.0-2.5 mg/kg is administered to quickly reach the target drug level.
• Maintenance Dose: Starting 12 hours after the loading dose, a regimen of 1.25-1.5 mg/kg is given every 12 hours.

A significant clinical consideration is that, unlike colistin, polymyxin B is not significantly cleared by the kidneys. Therefore, dose adjustments are generally not required for patients with renal impairment or those on renal replacement therapy.

Switching Therapy

Switching to polymyxin B therapy is typically reserved for severe infections caused by multidrug-resistant (MDR) Gram-negative pathogens when other antibiotic options are ineffective or contraindicated. The decision to switch should be based on antimicrobial susceptibility testing and a comprehensive clinical evaluation. The loading dose is crucial when initiating therapy to ensure therapeutic levels are achieved promptly.

Missed Dose

Polymyxin B is used for serious infections, making adherence to the dosing schedule critical. If a maintenance dose is missed, contact the prescribing physician or pharmacist for guidance immediately. Do not double the next dose. The appropriate action will depend on the time elapsed since the dose was due and the patient’s clinical condition.

Safety Alerts

Frequently Asked Questions (FAQ)

Why does the calculator use Actual Body Weight (ABW)?

Current international guidelines recommend using ABW for all polymyxin B dosing (both loading and maintenance) as it provides the most accurate basis for achieving therapeutic drug exposure.

Is a dose adjustment needed for poor kidney function?

No. Polymyxin B is primarily cleared through non-renal pathways. Therefore, dosage adjustments are not recommended for patients with renal impairment or those undergoing hemodialysis.

What is the difference between mg and IU for polymyxin B?

These are two different units of measurement for the drug. The standard conversion is 1 mg of polymyxin B base is equivalent to 10,000 International Units (IU).

When should the maintenance dose be started?

The first maintenance dose should be administered 12 hours after the start of the loading dose infusion.

Is there a maximum dose for polymyxin B?

While the calculator provides weight-based dosing, many institutions cap the total loading dose (e.g., at 200-250 mg) to mitigate toxicity risk in very large patients. Always consult institutional protocols.

Which equation is used to calculate creatinine clearance?

This tool uses the Cockcroft-Gault equation, which is a standard method for estimating renal function based on age, weight, sex, and serum creatinine.

Can this calculator be used for children?

No. This calculator is intended for adult patients only. Pediatric dosing for polymyxin B is different and requires specialized consultation.

What are the main side effects to monitor for during administration?

The most critical side effects are nephrotoxicity (kidney damage) and neurotoxicity (nerve damage). Close monitoring of renal function labs and neurological status is mandatory.

References

• Tsuji BT, Pogue JM, Zavascki AP, et al. International Consensus Guidelines for the Optimal Use of the Polymyxins: Endorsed by the American College of Clinical Pharmacy (ACCP), European Society of Clinical Microbiology and Infectious Diseases (ESCMID), Infectious Diseases Society of America (IDSA), International Society for Anti-infective Pharmacology (ISAP), Society of Critical Care Medicine (SCCM), and Society of Infectious Diseases Pharmacists (SIDP). Pharmacotherapy. 2019;39(1):10-39. doi:10.1002/phar.2209
• Nation RL, Garonzik SM, Thamlikitkul V, et al. Dosing guidance for polymyxin B in patients with renal impairment. J Antimicrob Chemother. 2017;72(1):191-198. doi:10.1093/jac/dkw377
• U.S. Food & Drug Administration (FDA). Polymyxin B for Injection – Prescribing Information. View PDF Label
• U.S. Food & Drug Administration (FDA). Drugs@FDA Database. Search Database