Alternative names, keywords


Samples required

Clotted blood (gold cap, 5 mL tube). Smaller tubes are available for paediatric samples.

14 - 28 days to steady state.

Test indications


Test information

An albumin-adjusted calculated free phenytoin is automatically reported with total phenytoin results.

Calculated free phenytoin = (measured total serum phenytoin in mg/L) / ((0.02 x (serum albumin in g/L)) + 0.1)

Source: Sheiner LB, Tozer TN. Clinical pharmacokinetics: the use of plasma concentrations of drugs. In: Melmon KL, Morrelli HF (editors). Clinical Pharmacology. Basic Principles in Therapeutics. MacMillan. 1978

In the rare cases where serum albumin binding is thought to be significantly abnormal, it may be possible to arrange referral of samples for direct measurement of free phenytoin.

Factors affecting test performance/ results

As with any assay employing mouse antibodies, the possibility exists for interference by human anti-mouse antibodies (HAMA) in the sample, which could cause falsely low results.

In rare instances (< 1 %), samples may contain unidentified component(s) which cause non-specific agglutination in this assay. These samples could give erroneously low phenytoin values. If this is suspected please contact Duty Biochemist.

In very rare cases, gammopathy, in particular type IgM (Waldenström’s macroglobulinemia), may cause unreliable results.

Results should always be assessed in conjunction with the patient’s medical history, clinical examination and other findings.

Reference range

Therapeutic range: 10 - 20 mg/L

Optimum serum concentration varies between individuals and may be outside this range.
Source: Roche Cobas® Phenytoin kit insert 08-2015, V10.0

Turnaround time

Same day (within 2h if urgent)


Biochemistry (Automation)