How many hours after taking Vyvanse (lisdexamfetamine) should a patient undergo the QbTest to correspond with the medication's peak effect?

Timing of QbTest After Vyvanse Administration

Administer the QbTest approximately 3.5 to 4 hours after the patient takes Vyvanse to capture peak medication effect. 1

Pharmacokinetic Rationale

The timing recommendation is based on the specific absorption and conversion profile of lisdexamfetamine:

• Lisdexamfetamine (Vyvanse) reaches peak plasma concentration (Tmax) at approximately 1 hour post-dose, but this represents the inactive prodrug 1
• The active metabolite d-amphetamine reaches peak concentration (Tmax) at approximately 3.5 hours post-dose in pediatric patients (ages 6-12) and 4.4 hours in adults 1, 2
• The prodrug undergoes rate-limited enzymatic hydrolysis in red blood cells to convert to active d-amphetamine, creating a delayed but sustained therapeutic effect 3, 1

Clinical Performance Data

Research on stimulant pharmacodynamics supports this timing window:

• Behavioral improvements with amphetamines occur later than initial absorption, with changes in cognitive performance (such as math performance) appearing approximately 1.5 hours after administration 4
• Peak pharmacodynamic effects on attention and cognitive tasks align with peak d-amphetamine concentrations, not with the prodrug peak 4
• The duration of therapeutic effect extends up to 13-14 hours post-dose, but peak effect corresponds to maximum plasma concentrations 5, 6

Practical Testing Protocol

For optimal QbTest administration:

• Schedule the test 3.5-4 hours after morning Vyvanse dose 1, 2
• Ensure the patient takes their usual prescribed dose on the testing day (do not adjust dosing for testing purposes) 4
• Food intake does not significantly affect d-amphetamine exposure but delays Tmax by approximately 1 hour 1—if the patient takes Vyvanse with a high-fat meal, consider scheduling the test 4.5-5 hours post-dose
• Avoid caffeine or other stimulants for at least 3 hours before testing to prevent confounding effects 4

Important Caveats

Several factors may influence individual timing:

• The chewable tablet formulation has a slightly later Tmax (4.4 hours) compared to capsules (3.5 hours) 1—adjust timing accordingly based on formulation
• Plasma elimination half-life of d-amphetamine is 8.6-9.5 hours in children and 10-11.3 hours in adults 1, meaning therapeutic effects persist well beyond peak concentrations
• Inter-subject variability in d-amphetamine pharmacokinetics is low (<25%) 1, making the 3.5-4 hour window reliable across most patients
• Patients with significant variations in red blood cell counts may have altered conversion rates, though substantial hydrolysis occurs even at 33% of normal hematocrit 1