Does the loading dose count as the first dose when taking a trough level before the fourth vancomycin (Vancomycin hydrochloride) dose?

Does the Loading Dose Count as the First Dose for Vancomycin Trough Timing?

Yes, the loading dose counts as the first dose when determining when to draw a trough level before the fourth vancomycin dose. 1, 2

Rationale for Trough Timing

• The Infectious Diseases Society of America recommends obtaining trough serum vancomycin concentrations just before the fourth dose, at steady-state conditions 1
• Steady-state achievement is variable but occurs approximately just before the fourth dose 1
• More recent guidelines specify that serum trough concentrations should be obtained at steady state conditions, prior to the fourth or fifth dose 2

Counting Methodology

• When a loading dose is administered (typically 25-30 mg/kg for seriously ill patients), it is counted as dose #1 2
• The subsequent maintenance doses are then counted as doses #2, #3, and #4 2
• Therefore, if a patient receives a loading dose followed by three maintenance doses, the trough should be drawn before the fourth total dose (which is the third maintenance dose) 1, 2

Clinical Application Algorithm

If loading dose given:

• Loading dose = Dose #1
• First maintenance dose = Dose #2
• Second maintenance dose = Dose #3
• Draw trough before third maintenance dose (fourth total dose) 1, 2

If no loading dose given:

• First maintenance dose = Dose #1
• Second maintenance dose = Dose #2
• Third maintenance dose = Dose #3
• Draw trough before fourth maintenance dose 1, 2

Important Timing Considerations

• The trough should be drawn immediately before the next scheduled dose, not simply at a fixed time interval after the previous dose 2
• For patients receiving vancomycin every 12 hours, this means drawing the trough approximately 12 hours after the previous dose, but the critical factor is timing it immediately before the next dose 2
• The goal is to capture the true trough concentration at steady state to guide dosing adjustments 1, 3

Common Pitfalls to Avoid

• Do not exclude the loading dose from your count - this will result in drawing the trough too late, potentially missing early opportunities to adjust dosing 2
• Do not draw the trough too early (before the third dose) - steady state may not yet be achieved, leading to inaccurate interpretation 1
• Failing to draw the trough immediately before the next dose can result in falsely elevated or decreased levels depending on timing 2