About this Calculator

The Rifapentine Dose Calculator (LTBI) is a clinical support tool designed to help healthcare professionals determine appropriate dosing for two short-course rifapentine-based regimens for latent tuberculosis infection. It operationalizes recommendations from the U.S. Centers for Disease Control and Prevention (CDC) for the 3HP and 1HP regimens.

Outputs

After entering the patient’s information, the calculator provides the following outputs:

• Rifapentine (RPT) Dose: The calculated dose in milligrams.
• Isoniazid (INH) Dose: The calculated dose in milligrams.
• Tablet Formulation: A practical guide on how to achieve the dose using common tablet strengths (e.g., rifapentine 150 mg, isoniazid 100 mg and 300 mg).
• Administration Schedule: The frequency (weekly or daily) and total duration of the selected regimen.
• Clinical Notes: Important context-specific alerts, such as age or weight restrictions and notifications when maximum recommended doses are reached.

How to Use

The calculator requires three inputs to determine the correct dosing schedule:

1. Patient Weight: Enter the patient’s body weight. You can use either kilograms (kg) or pounds (lbs). The tool will automatically convert lbs to kg for calculations.
2. Patient Age: Enter the patient’s age in years. This is critical as some regimens have age-based recommendations.
3. Treatment Regimen: Select either the 3HP (weekly) or 1HP (daily) regimen from the dropdown menu.

Dosing Overview

The calculator uses established weight-based and mg/kg dosing strategies for two primary regimens:

• 3HP Regimen (3 months weekly): This regimen consists of 12 weekly doses of Isoniazid and Rifapentine. It is generally recommended for adults and children aged 2 years and older. Dosing is based on weight brackets, with a maximum dose of 900 mg for both drugs.
• 1HP Regimen (1 month daily): This regimen involves 28 daily doses of Isoniazid and Rifapentine. It is typically recommended for individuals aged 13 years and older. Dosing is calculated based on body weight (Rifapentine ~10 mg/kg, Isoniazid ~5 mg/kg), with maximum daily doses of 600 mg for rifapentine and 300 mg for isoniazid.

Switching Regimens

Switching from one LTBI treatment regimen to another should be a clinical decision based on factors like adverse effects or patient preference. If a regimen is stopped due to an adverse event, the clinician must assess if the event contraindicates drugs in the new regimen. In most cases, switching regimens requires starting the new regimen from the beginning; doses from a prior regimen do not count toward completion of the new one.

Missed Dose Guidance

Management of missed doses depends on the regimen:

• For 3HP (Weekly): All 12 doses should be completed within a 16-week period. If a patient goes more than 4 weeks without a dose, they should be medically re-evaluated before restarting the entire 12-dose course.
• For 1HP (Daily): The full course of 28 doses should be completed within a 42-day (6-week) period. If this window is exceeded, the patient should be re-evaluated to determine the best course of action.

Safety Alerts

• Hepatotoxicity: Both isoniazid and rifapentine can cause liver injury. Baseline and follow-up liver function tests are recommended for patients with liver disease or other risk factors. Patients should be counseled to report any symptoms of hepatotoxicity (nausea, fatigue, jaundice, abdominal pain).
• Drug Interactions: Rifapentine is a potent inducer of CYP450 enzymes and can significantly reduce the effectiveness of many medications, including hormonal contraceptives, certain anticoagulants, and antiretrovirals. A thorough medication review is essential before starting treatment.
• Body Fluid Discoloration: Patients should be informed that rifapentine will cause a harmless orange-red discoloration of body fluids, including urine, sweat, and tears. This can permanently stain soft contact lenses.

Frequently Asked Questions

1. Why does the calculator recommend specific tablet counts?
   To simplify administration. It breaks down the total milligram dose into the number of available tablets (e.g., 150 mg for rifapentine), helping to ensure accurate dosing.
2. Can the 3HP regimen be used for a patient weighing less than 10 kg?
   The 3HP regimen is not recommended for children weighing less than 10 kg. The calculator will display a note advising consultation with pediatric guidelines for these patients.
3. What should I do if a patient’s weight is on the border between two dosing brackets for the 3HP regimen?
   In general, use the dosing bracket corresponding to the patient’s actual weight. If there is concern, or if the weight has been fluctuating, clinical judgment should be used. Consulting a pharmacist or infectious disease specialist is advisable.
4. Is the 1HP regimen safe for children under 13?
   The 1HP regimen is primarily studied and recommended for individuals aged 13 and older. The calculator flags this for younger patients, and its use in this population should be based on expert consultation.
5. Should rifapentine be taken with food?
   Yes, rifapentine absorption is increased when taken with a meal. Patients should be advised to take their doses with food.
6. How does rifapentine affect hormonal birth control?
   Rifapentine can make hormonal contraceptives (pills, patches, rings) less effective. Patients using these methods should be counseled to use an alternative or additional non-hormonal barrier method of contraception during and for one month after treatment.
7. Does the orange-red discoloration of body fluids go away after treatment?
   Yes, the discoloration is temporary and will resolve after the medication is stopped.
8. What are the most common side effects?
   Besides body fluid discoloration, common side effects can include rash, headache, and flu-like symptoms. Severe reactions like liver injury or hypersensitivity are rare but require immediate medical attention.

References

• CDC: Treatment for Latent TB Infection (LTBI)
• MMWR: Guidelines for the Treatment of Latent Tuberculosis Infection (2020)
• Priftin (Rifapentine) Prescribing Information (FDA)
• Isoniazid Tablets Prescribing Information (FDA)