Precautions

To ensure the reliability of TDM, it’s essential to address pre-analytical and analytical factors, including special precautions, specimen handling, and interfering substances

Special Precautions

• Patient Identification
  • Accurate patient identification is essential to prevent errors
  • Use two unique identifiers (e.g., name and date of birth)
• Timing of Specimen Collection
  • Collect specimens at the correct time relative to the drug administration schedule
    • Trough levels: Collected immediately before the next dose
    • Peak levels: Collected at a specific time after drug administration
  • Record the date and time of drug administration and specimen collection
• Medication History
  • Obtain a complete medication history from the patient
  • Document all prescription and over-the-counter medications, as well as any herbal supplements
• Clinical Information
  • Gather relevant clinical information, such as the patient’s age, weight, renal function, liver function, and concurrent medical conditions
• Chain of Custody
  • Maintain a clear chain of custody for all specimens
  • Document the date, time, and initials of each person handling the specimen
• Informed Consent
  • Obtain informed consent from the patient before collecting specimens for TDM
• Safety Precautions
  • Follow standard safety precautions when handling blood and other biological specimens
  • Wear gloves, gowns, and eye protection

Specimen Collection

• Specimen Type
  • Serum or plasma is commonly used for TDM
  • Whole blood is used for some immunosuppressant drugs (e.g., tacrolimus, cyclosporine)
• Collection Tubes
  • Use appropriate collection tubes for each drug
    • Serum separator tubes (SST)
    • EDTA tubes
    • Heparin tubes
  • Follow the manufacturer’s instructions for tube selection
• Order of Draw
  • Follow the correct order of draw to prevent cross-contamination
  • Consult the CLSI guidelines for the correct order of draw
• Collection Technique
  • Use proper venipuncture technique to minimize hemolysis
  • Avoid prolonged tourniquet time
• Specimen Volume
  • Collect sufficient volume to perform the assay and allow for repeat testing
  • Follow the laboratory’s guidelines for minimum volume requirements
• Labeling
  • Label specimens immediately and accurately with patient information, date, and time of collection

Specimen Processing

• Centrifugation
  • Centrifuge specimens promptly after collection
  • Follow the manufacturer’s instructions for centrifugation speed and time
• Separation
  • Separate serum or plasma from cells within the specified time frame
  • Prevent hemolysis
• Aliquotting
  • Aliquot specimens into appropriate containers
  • Use clean, dry containers
• Storage
  • Store specimens at the appropriate temperature
    • Refrigeration (2-8°C): For short-term storage
    • Freezing (-20°C or -80°C): For long-term storage
  • Follow the manufacturer’s instructions for storage conditions
• Thawing
  • Thaw specimens properly before testing
  • Thaw at room temperature or in a refrigerator, and mix gently

Troubleshooting

• High Drug Results
  • Pre-analytical: Specimen collected at the wrong time, medication administration error, contamination
  • Analytical: Incorrect calibration, reagent deterioration, interfering substances, cross-reactivity
• Low Drug Results
  • Pre-analytical: Specimen collected at the wrong time, improper storage, medication error, dilution
  • Analytical: Incorrect calibration, reagent deterioration, interfering substances, assay drift
• Inconsistent Results
  • Pre-analytical: Patient variability, specimen handling errors, timing errors
  • Analytical: Assay variability, interfering substances, cross-reactivity
• Quality Control Failures
  • Pre-analytical: QC material improperly stored, QC material expired
  • Analytical: Incorrect calibration, reagent deterioration, instrument malfunction
  • Action: Investigate the cause of the failure and take corrective action

Interfering Substances

• Hemolysis
  • Interference: Releases intracellular components that can interfere with immunoassays
  • Mitigation: Avoid hemolysis during collection and processing
• Lipemia
  • Interference: Turbidity from high lipid concentrations can interfere with spectrophotometric assays
  • Mitigation: Use lipemia clearing techniques or consider methods less affected by turbidity
• Bilirubin
  • Interference: High bilirubin levels can interfere with spectrophotometric assays
  • Mitigation: Use methods less susceptible to bilirubin interference
• Drugs
  • Interference: Other drugs can directly interfere with assays or affect drug levels
  • Mitigation: Be aware of patient’s medication list and potential drug interactions
• Heterophile Antibodies
  • Interference: Bind to assay antibodies, causing falsely high or low results
  • Mitigation: Use blocking reagents or heterophile antibody blocking tubes
• Cross-Reactivity
  • Interference: Assay antibodies may cross-react with structurally similar compounds
  • Mitigation: Use highly specific antibodies and be aware of potential cross-reactants
• Matrix Effects
  • Interference: Components in the sample matrix can affect the assay results
  • Mitigation: Use matrix-matched calibrators and quality control materials
• Endogenous Substances
  • Interference: Digoxin-like immunoreactive substances (DLIS) can interfere with digoxin assays
  • Mitigation: Use methods less susceptible to DLIS interference

Factors Specific to Different Drug Classes

• Aminoglycosides
  • Renal function: Monitor serum creatinine and creatinine clearance
  • Interfering antibiotics: Certain penicillins can inactivate aminoglycosides in vitro
• Cardioactive Drugs
  • Electrolyte levels: Monitor potassium, magnesium, and calcium
  • Cardiac function: Assess ECG for arrhythmias
  • Digoxin-like immunoreactive substances (DLIS): Especially in neonates and patients with renal failure
• Anticonvulsants
  • Protein binding: Monitor free drug levels in patients with altered protein binding
  • Drug interactions: Phenobarbital is a CYP enzyme inducer
• Antidepressants
  • CYP2D6 genotype: Consider genetic testing to predict drug metabolism
  • Drug interactions: Many antidepressants are CYP enzyme inhibitors
• Immunosuppressants
  • Hematocrit levels: Use whole blood specimens and correct for hematocrit
  • Calcineurin inhibitors: Monitor renal function and blood pressure
  • Drug interactions: Many drugs can affect CYP3A4-mediated metabolism

Key Terms

• Pre-Analytical: Processes before sample analysis
• Analytical: Processes during sample analysis
• Post-Analytical: Processes after sample analysis
• Interfering Substance: Affects test accuracy
• Calibration: Adjusting an instrument for accurate readings
• Quality Control: Measures to ensure test accuracy and precision
• Homogenous assay: Assay where the antigen/antibody reaction is measured without a separation step
• Heterogenous assay: Assay where the antigen/antibody reaction is measured with a separation step
• Hemolysis: Breakdown of red blood cells
• Lipemia: Excess lipids in the blood