Sir: Coates (Psychiatric Bulletin, May 2001, 25, 195) is correct in his assertion that a positive result (that is, the detection of olanzapine in urine) is open to various interpretations. This is the very reason why the value of this means of assessment of compliance is so questionable in clinical practice at present and is unlikely to prove better in the future, particularly for low doses of oral medication.

The quantitative assessment of urinary olanzapine is limited, as follows: 7% of the dose of olanzapine is excreted unchanged in urine. As urine concentrations vary with the volume of fluid intake, which can vary greatly in any individual from day-to-day, exact assessment of urine concentration would necessitate specific timed and total urine collection. Compliance with such a protocol is a most unlikely scenario, particularly in the very patients whose nonadherence is suspected. Even using creatinine concentrations for normalisation is inexact, and must require a number of assumptions to be made.

Coates' other scenario of a negative result, that is one that detects no olanzapine in the urine, is also debatable; even if no olanzapine could be detected in the urine one might speculate that no olanzapine had been taken recently, however even this is questionable given issues of assay sensitivity, drug stability in urine and fluid volume intake.

Taken together, none of the above results will deliver the level of precision of measurement that may be required for such purposes as mental health review tribunals as Coates has previously suggested (Psychiatric Bulletin, 2000, 24, 316).

On a more general point, awareness of a patient's adherence to medication is a major issue in the treatment of psychiatric patients, particularly if Mental Health Act legislation is to change to allow the community treatment of patients. Knowledge of a patient's true adherence is extremely difficult particularly in clinical practice.

Until technological developments improve for the quantitative assessment of drugs in body fluids other more direct observations of medication intake may be the best substitute for assessing adherence to medication. For example, one demonstrably successful method of achieving compliance and levels of compliance with methadone prescribing has been to directly observe patients taking their medication in liquid form. A similar trial is planned that involves community pharmacists observing community patients taking antipsychotic medications when they visit the pharmacy on a daily basis in return for a small daily reward.