3. CONVERSION FROM INTRAVENOUS TO ORAL DOSING

Conversion from Intravenous to Oral Dosing

Conversion from intravenous (IV) to oral (PO) dosing is a fundamental component of clinical pharmacokinetics and modern hospital antimicrobial stewardship programs. The IV-to-PO switch strategy promotes safe, effective, and cost-efficient medication use by transitioning patients from parenteral therapy to oral therapy once they are clinically stable. This shift not only reduces the risks associated with IV therapy but also enhances patient comfort and supports faster hospital discharge.

Introduction to IV-to-PO Conversion

Many medications have excellent oral bioavailability and achieve therapeutic concentrations comparable to those achieved with IV administration. When a patient no longer requires parenteral access for clinical reasons, switching to oral therapy offers multiple advantages without compromising therapeutic outcomes.

The success of IV-to-PO conversion depends on understanding differences in bioavailability, pharmacokinetic parameters, patient status, and the clinical condition being treated.

Why Convert IV to Oral Dosing?

Key reasons for IV-to-PO conversion include:

  • Reduced risk of IV-related complications such as phlebitis, infection, and thrombosis
  • Lower healthcare costs due to less use of IV supplies and nursing time
  • Improved patient comfort by removing IV lines
  • Earlier discharge because oral therapy allows outpatient management
  • Enhanced mobility without IV poles or lines limiting movement

Pharmacokinetic Basis of IV-to-PO Switching

IV administration delivers 100% bioavailability, while oral bioavailability varies by drug. Understanding these differences is essential for correct dose conversion.

Bioavailability (F)

Bioavailability is the fraction of an oral dose that reaches systemic circulation unchanged.

F = (AUCoral / AUCIV) × (DoseIV / Doseoral)

To maintain the same systemic exposure, the dose must be adjusted for drugs with low oral bioavailability.

Criteria for Switching from IV to Oral Therapy

Before switching, the patient must meet all the following criteria:

1. Clinical Stability

  • Afebrile for 24–48 hours
  • Hemodynamically stable
  • Improvement in infection markers (if applicable)

2. Functional Gastrointestinal Tract

  • Ability to swallow
  • No vomiting or uncontrolled diarrhea
  • Intact absorption capacity

3. Oral Drug Availability

  • An effective oral formulation must exist
  • Oral dose should achieve therapeutic levels

4. No Contraindication to PO Therapy

  • Not on strict NPO orders
  • No severe malabsorption syndromes
  • No drug-food restrictions that cannot be managed

Types of IV-to-PO Conversion

There are three primary types of conversion:

1. Same-Dose Conversion

Used when oral bioavailability is high (> 90%).

Example: Levofloxacin 500 mg IV = 500 mg PO

2. Dose-Equivalent Conversion

Used when oral dosing differs from IV dosing due to moderate bioavailability.

3. Therapeutic Substitution

Used when an oral alternative with similar efficacy is substituted for an IV-only agent.

Dose Conversion Formula

The following formula helps calculate equivalent oral doses:

Doseoral = (DoseIV × FIV) / Foral

Since IV bioavailability is always 1 (or 100%), the formula simplifies to:

Doseoral = DoseIV / Foral

This enables precise dose selection for drugs with lower oral bioavailability.

Examples of High-Bioavailability Drugs Suitable for Direct Switch

Drugs with oral bioavailability ≥ 80% are ideal candidates for same-dose switching.

  • Levofloxacin
  • Linezolid
  • Doxycycline
  • Metronidazole
  • Fluconazole
  • Trimethoprim–sulfamethoxazole
  • Clindamycin

Drugs Not Suitable for IV-to-PO Conversion

  • Aminoglycosides
  • Vancomycin
  • Amphotericin B
  • Drugs requiring continuous infusion
  • Drugs with negligible oral absorption

Clinical Situations Favoring IV-to-PO Conversion

IV-to-PO switch is often beneficial in:

  • Pneumonia once stable
  • Urinary tract infections
  • Skin and soft tissue infections
  • Sepsis recovering phase
  • Chronic therapy (e.g., antifungals, antivirals)

Therapeutic Drug Monitoring (TDM) Considerations

For drugs requiring TDM, conversion should be approached cautiously.

Consider the following:

  • Bioavailability variability between patients
  • Risk of subtherapeutic levels
  • Drug–food interactions altering absorption
  • Changes in protein binding

For narrow therapeutic index drugs, plasma level monitoring may be required after switching to oral therapy.

Advantages of IV-to-PO Conversion

Benefits to patients and hospital systems include:

  • Fewer IV-related complications
  • Improved patient comfort and mobility
  • Reduced nursing workload
  • Shorter hospital length of stay
  • Cost savings without affecting outcomes

Detailed Notes:

For PDF style full-color notes, open the complete study material below:

PATH: PHARMD/ PHARMD NOTES/ PHARMD FIFTH YEAR NOTES/ CLINICAL PHARMACOKINETICS AND PHARMACOTHERAPEUTIC DRUG MONITORING (TDM)/ CONVERSION FROM INTRAVENOUS TO ORAL DOSING.

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