Phenobarbitone (Gardinal)

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Drug level monitoring.

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Phenobarbitone (Gardinal) - Comprehensive Medical Test Guide

Why is it done?
- Test Purpose: Measures serum concentration of phenobarbitone, a long-acting barbiturate medication used as an anticonvulsant and sedative
- Primary Indications: Therapeutic drug monitoring in patients receiving phenobarbitone for epilepsy, seizure disorders, or chronic sedation
- Assessment of Toxicity: Evaluation of potential drug toxicity or overdose symptoms including excessive sedation, ataxia, or behavioral changes
- Compliance Monitoring: Verification of medication adherence and proper dosing in patients with seizure disorders
- Timing of Testing: Typically performed at steady-state (5-7 days after dose initiation), during dose adjustments, or when toxicity is suspected
Normal Range
- Therapeutic Range: 15-40 μg/mL (or 65-170 μmol/L)
- Unit of Measurement: Micrograms per milliliter (μg/mL) or micromoles per liter (μmol/L)
- Subtherapeutic Level: <15 μg/mL indicates inadequate serum concentration; seizure control may be insufficient
- Toxic Level: >40 μg/mL represents potential toxicity with risk of severe adverse effects
- Critical Level: >60 μg/mL requires immediate clinical intervention due to risk of severe toxicity, respiratory depression, or coma
- Interpretation Summary: Normal represents the optimal zone for seizure control with minimal adverse effects; results outside this range require clinical evaluation and possible dose adjustment
Interpretation
- Low Levels (< 15 μg/mL): Indicates subtherapeutic concentration; patient may experience breakthrough seizures or inadequate seizure control. Consider dose increase or improved compliance.
- Therapeutic Levels (15-40 μg/mL): Optimal range for seizure management with acceptable side effect profile. Continue current dosing and maintain therapeutic monitoring schedule.
- Toxic Levels (40-60 μg/mL): Associated with adverse effects including lethargy, ataxia, nystagmus, dysarthria, cognitive impairment, and behavioral changes. Dose reduction recommended; monitor clinical symptoms closely.
- Critical Levels (> 60 μg/mL): Severe toxicity risk with potential complications including profound CNS depression, respiratory compromise, hypotension, cardiovascular collapse, and coma. Requires immediate intervention and possible hospitalization.
- Factors Affecting Results: Timing of sample collection (peak vs. trough), patient compliance, drug interactions (enzyme inducers/inhibitors), liver function, renal disease, age, pregnancy, and individual metabolic variations
- Drug Interactions: Phenobarbitone is a potent enzyme inducer; levels may be affected by other medications and may affect metabolism of concurrent drugs
- Clinical Significance: Test is essential for optimizing seizure control while minimizing toxicity; particularly important in patients with difficult-to-control epilepsy or those at risk for drug interactions
Associated Organs
- Primary Organ Systems: Central nervous system (brain and spinal cord), where phenobarbitone exerts its anticonvulsant effect
- Hepatic System: Liver metabolizes phenobarbitone; hepatic dysfunction significantly affects drug levels and clearance
- Renal System: Minor role in drug elimination; renal disease may increase drug accumulation
- Conditions Associated with Abnormal Results: Epilepsy, generalized tonic-clonic seizures, partial seizures, status epilepticus, febrile seizures in infants
- Potential Complications from Toxicity: Respiratory depression, hypotension, bradycardia, circulatory collapse, cerebral edema, hepatotoxicity, Stevens-Johnson syndrome, toxic epidermal necrolysis, blood dyscrasias
- Long-term Effects: Chronic use associated with cognitive effects, behavioral changes, vitamin D deficiency, osteoporosis, and increased risk of suicidal ideation
Follow-up Tests
- Repeat Phenobarbitone Level: 5-7 days after dose adjustment or initiation; then at intervals determined by clinical response (typically every 3-6 months for stable patients)
- Liver Function Tests (LFTs): Baseline and periodic monitoring; ALT, AST, alkaline phosphatase, bilirubin to assess hepatic function and metabolism capacity
- Renal Function Panel: Creatinine and BUN particularly important in elderly patients or those with renal disease
- Complete Blood Count (CBC): Monitor for blood dyscrasias including anemia, agranulocytosis, and thrombocytopenia; baseline and annually
- Vitamin D and Calcium Levels: Assess for deficiencies associated with long-term phenobarbitone use; important in patients on chronic therapy
- Concurrent Drug Levels: Monitor levels of other medications (warfarin, phenytoin, valproic acid) as phenobarbitone is a potent enzyme inducer
- EEG (Electroencephalogram): Assess seizure control and brain electrical activity; recommended periodically or when seizures persist despite therapeutic levels
- Bone Density Scan (DEXA): Consider in patients with long-term use to assess for osteoporosis risk
Fasting Required?
- Fasting Status: NO fasting is required for phenobarbitone level testing
- Sample Collection Timing: Trough level (just before next dose) is preferred for routine monitoring; peak level (2-4 hours post-dose) may be ordered in specific clinical situations
- Medication Instructions: Continue taking phenobarbitone at the regular scheduled time; do NOT skip doses before testing
- No Medication Adjustments: Do not alter or discontinue other medications without medical consultation; report all current medications and supplements to healthcare provider
- Alcohol Restriction: Avoid alcohol consumption for 24 hours before testing as it may affect drug metabolism and results
- General Preparation: Standard blood draw procedure; wear comfortable clothing with loose sleeves for easy access; arrive on time for appointment
- Important Information: Inform laboratory staff of exact timing of last phenobarbitone dose to ensure appropriate test interpretation; notify healthcare provider of any recent medication changes or new symptoms

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