Common decision limits — The need for harmonised immunoassays

• Common decision limits are recommended in many clinical guidelines.
• Impact of method-related differences for use of common decision limits may be severe.
• Improving method comparability is a pre-requisite for use of common decision limits.

The main aim of clinical guidelines is to encourage the best clinical outcome for patients and the best use of resources, no matter where patients are investigated or managed. Where guidelines incorporate decision limits based on levels of analytes in serum, plasma or urine these may determine whether or not to treat or may be used to tailor further treatment. Consideration should be given to the effect of method-related differences in results when implementing common decision limits. Available evidence suggests that for some analytes the implications for the patient may be serious, e.g. in terms of missed biopsies or unnecessary prostatic biopsies when prostate specific antigen is measured. Major causes of between-method differences are reviewed and means of addressing them considered.