Performing Pharmacokinetic Testing with a Peripheral IV
Draw blood samples for PK testing from a separate peripheral IV site—never from the same line used for drug infusion—using a 20-22 gauge catheter with precise timing documentation and appropriate sample handling protocols.
Essential Sampling Principles
Site Selection and Catheter Specifications
Use a dedicated peripheral IV line for blood sampling that is completely separate from the infusion line. Drawing from the same line that delivers the drug will contaminate samples with residual medication, invalidating PK results 1.
A 20-22 gauge peripheral IV catheter is appropriate for PK blood sampling in most patients, as these sizes provide adequate blood flow while minimizing vessel trauma 2, 3.
Preferentially place the sampling catheter in the forearm or antecubital fossa, as these sites have the lowest failure rates (50-53%) compared to other locations 4.
Critical Timing and Documentation Requirements
Record the start time of the drug infusion using calendar time (month, day, hours, minutes) with maximum precision 5.
Document each blood sampling time with equal precision using the same calendar time format to enable accurate PK calculations 5.
Obtain samples at predetermined time points based on the drug's half-life and expected distribution phase. For example, hemophilia factor concentrates require samples at least 12 hours apart over 4-48 hours, with the 24-hour sample providing the most information 5.
Include a pre-infusion baseline sample when the patient receives the infusion in a clinical setting to establish true baseline drug levels 5.
Patient-Specific Dosing Considerations
Weight-Based Adjustments
Calculate lean body weight (height-based) rather than total body weight for obese patients when determining drug dosing and interpreting PK parameters, as this prevents overestimation of drug clearance 5, 6.
For pediatric patients weighing ≥40 kg, transition to adult dosing protocols rather than continuing weight-based calculations that might exceed maximum safe doses 7.
Renal Impairment Modifications
Use the Cockcroft-Gault equation with ideal body weight (not actual weight) in obese patients to calculate creatinine clearance and avoid overdosing of renally cleared drugs 6.
For creatinine clearance <30 mL/min, reduce dosing frequency of renally eliminated drugs (e.g., oseltamivir changes from twice daily to once daily) 6.
Adjust doses of specific medications based on creatinine clearance ranges: For example, dofetilide requires 500 μg BID for CrCl >60 mL/min, 250 μg BID for 40-60 mL/min, 125 μg BID for 20-40 mL/min, and is contraindicated below 20 mL/min 5.
Hepatic Impairment Considerations
For severe hepatic dysfunction, reduce doses of hepatically metabolized drugs (e.g., rimantadine limited to 100 mg/day) with close monitoring for adverse effects 6.
Monitor INR more frequently in patients on warfarin following bariatric surgery, as absorption changes may require dose reductions of approximately 30% from pre-surgery levels 5.
Age-Related Factors
Repeat PK assessments in children over time, as drug clearance and volume of distribution change with growth and development 5.
Reduce initial doses by 20% or more in patients >60 years or those with ASA physical status III or greater when using sedatives like midazolam during procedures 5.
Drug Interaction Monitoring
Document all concurrent medications, particularly those affecting drug metabolism: For example, H2-receptor antagonists increase midazolam bioavailability by approximately 30% 5.
Record the times and doses of the last three infusions of the study drug to account for accumulation in PK modeling 5.
Laboratory Handling and Reporting
Sample Processing Standards
Use appropriate laboratory assays for the specific drug formulation: Standard clotting assays work for conventional factor concentrates, but extended half-life products may require specialized chromogenic assays 5.
Report measurements in International Units per milliliter (IU/mL) to maintain consistency across laboratories, converting from alternative units (IU/dL or percentage activity) as needed 5.
Record values below the limit of quantification (BLQ) rather than discarding them, as ignoring BLQ data introduces bias into individual PK estimates 5.
Additional Covariate Documentation
- Measure and record height to calculate lean body weight, blood group, hematocrit, and von Willebrand factor antigen levels (for factor VIII concentrates), as these covariates influence PK parameters 5.
Common Pitfalls to Avoid
Never draw PK samples from the same IV line used for drug administration, even after flushing, as residual drug in the catheter dead space will falsely elevate measured concentrations 1.
Avoid using 22-24 gauge catheters for prolonged sampling protocols, as these smaller sizes have the highest failure rate (60.5%) and may require replacement mid-study 4.
Do not rely on rapid IV push administration of midazolam or other sedatives during sampling procedures, as faster administration increases the risk of apnea occurring up to 30 minutes after the last dose 5.
Avoid the wrist/hand for IV placement when multiple samples are needed, as these sites have 34% higher failure rates compared to forearm sites 3.
Do not assume standard dosing applies to extremes of body weight: Underweight patients (<60 kg) and obese patients (BMI >30) require specific dose adjustments for many anticoagulants and other drugs 5.