Professional reference. Tube requirements differ between laboratories and analysers. Your lab’s handbook is the authority.
Quick reference
- Sample: serum in a gel (SST) or plain red-top tube, or as your lab specifies; centrifuge within about 2 hours.
- Timing: trough, immediately before the morning dose, after about 3–5 days on a stable dose.
- Usual target (total valproate): about 50–100 mg/L for epilepsy; up to about 125 mg/L for acute mania in some protocols.
Why the tube can matter
Gel separator tubes (SST, “gold top”, “tiger top”) make serum separation easy, but the gel can absorb some lipophilic drugs over time, lowering measured levels, especially if the sample sits for hours before centrifugation. This was historically documented for drugs such as phenytoin, carbamazepine and some tricyclics. Modern gels absorb less, and many labs have validated SST tubes for valproate and other antiepileptics, but some still specify plain red tubes. Lithium-heparin (green) tubes give plasma, which is acceptable for valproate at many labs; they must never be used for lithium levels.
Timing and interpretation
- Steady state: valproate’s half-life is about 9–16 hours in adults (shorter with enzyme-inducing drugs), so steady state is reached in about 3–5 days.
- Trough: draw before the next dose; with extended-release divalproex (Depakote ER), timing matters less, but be consistent.
- Record: the time of the last dose and the time of sampling on the request.
- Free valproate: valproate is highly protein-bound. In low albumin (liver disease, malnutrition, pregnancy, elderly) or kidney failure, the total level underestimates the active free fraction; ask for a free level if toxicity is suspected.
- Treat the patient: levels guide but don’t replace clinical assessment (seizure control, sedation, tremor, ammonia, LFTs, platelets).
Tube and timing for other common levels
| Drug |
Usual sample (check your lab) |
Timing |
| Valproate |
Serum (red or SST) or heparin plasma |
Trough |
| Carbamazepine |
Serum (red preferred) |
Trough |
| Phenytoin |
Serum (red preferred) |
Trough; correct for albumin or request free level |
| Lithium |
Serum; never lithium-heparin |
12 hours post-dose |
| Digoxin |
Serum or heparin plasma |
≥6 hours post-dose |
| Vancomycin |
Serum or plasma per lab |
Trough or AUC sampling per protocol |
| Tacrolimus, ciclosporin |
EDTA whole blood (lavender) |
Trough |
More timing notes and other practical references are in the clinical pharmacy notes.
Frequently asked questions
What colour tube is used for a vancomycin trough?
Commonly serum (red or gold) or lithium-heparin plasma (green), depending on the lab. Many hospitals now use AUC-guided dosing with two timed levels rather than a single trough.
Can a Depakote level be drawn from an IV line?
Avoid drawing from a line through which valproate is being infused. If a line must be used, follow local policy for flushing and discard volume.
What if the sample was drawn after the dose?
Record the actual time; a post-dose level will be higher than the trough. Interpret with caution or repeat at the correct time.
Sources and further reading
CanforaMedicine Guide
