pathology services handbook - St George's Healthcare NHS Trust

4.16 SAMPLE COLLECTION PROCEDURE FOR THERAPEUTIC DRUGS
Record on Request Form:
1. Sample collection time and date
2. Time and date of last dose
3. Current dosage
4. Concurrent medication and relevant medical information
Time of sample in relation to dose is important.
The trough level, collected immediately before the next dose (at steady state), is most
reproducible but the peak level may also be useful in some circumstances.
If Sodium Valproate is to be measured, the sample should be taken at a standard time,
preferably before the morning dose (Note: the value of valproate monitoring is controversial
and is rarely helpful, except when compliance is in doubt). Toxic effects show no clear
relationship to serum concentration.
Pre-dose (trough) Carbamazepine, Valproate (morning), Phenytoin (if
given as one large dose), Theophylline (peak also
useful)
6 hours post-dose Digoxin
12 hours post-dose Lithium
4.17 USUAL THERAPEUTIC (TARGET) RANGES
Therapeutic ranges are only a guide. Some patients may be well controlled with serum
concentrations below or above the therapeutic ranges given below. Factors affecting serum
concentrations should also be taken into account.
Adults Neonates
Carbamazepine 4 - 12 mg/L (single therapy)
4 - 8 mg/L (multiple therapy)
-
Digoxin 0.5 - 2.0 ug/L Up to 3.0 ug/L
Lithium 0.4 - 1.0 mmol/L -
Phenobarbitone 10 - 40 mg/L 13 - 30 mg/L
Phenytoin 5 - 20 mg/L 6 - 14 mg/L
Theophylline 10 - 20 mg/L 5-11 mg/L
Sodium Valproate 50 - 100 mg/L* -
*No definitive data to support a therapeutic range.
4.18 EFFECT OF AGE IN THERAPEUTIC DRUG MONITORING
Factor Effect
Reduced drug metabolism Serum phenobarbitone, theophylline increased in the elderly
Decreased albumin Serum phenytoin decreased in the elderly
Decreased GFR Reduced dosage for digoxin, lithium required in neonates
and the elderly
Hypokalaemia (e.g. Digoxin toxicity more likely
diuretics)
Variation in half life:
e.g. neonates
infants 1 month
children 1-15 years
adults
Phenytoin t ½:
30-60 hours
2-7 hours
2-29 hours
20-30 hours
Pathology Services Handbook v3.2.2 - February 2013 29 of 81