Hormones and other analytes that are requested in the investigation of endocrine abnormalities are routinely measured using automated immunoassays in clinical laboratories. Assay kit inserts usually detail interference in terms of % cross reactivity, but this information is not always communicated to the clinician. Another complicating factor is that different assay platforms show different degrees of interference, therefore clinicians need to be aware of the source of their patients results. UK NEQAS is an NHS External Quality Assessment provider of clinical specimens available for clinical laboratories. This allows laboratories to have confidence in the quality of the results that they provide to their users. UK NEQAS is uniquely placed to challenge the different analytical platforms with the same specimen and UK NEQAS prides itself in also providing an educational service which shares and improves knowledge. This poster explores a number of well-known drug interferences in immunoassays across different analytical platforms; for example prednisolone and metyrapone in cortisol assays, norethisterone in testosterone assays and biotin in immunoassays in general. Prednisolone is known to cross react in Cortisol immunoassays and we show at a therapeutic concentration of prednisolone, the % cross reactivity ranges from 55% in the Siemens ADVIA Centaur cortisol assay, compared to 6% in the Roche Gen II cortisol assay. All the most commonly used immunoassays are all affected to some degree. There are cases where not all immunoassays are affected; these include 11-Deoxycortisol interference (Metyrapone blocks cortisol biosynthesis and leads to an increase in circulating 11-Deoxycortisol levels), Norethisterone interference in testosterone assays and Biotin interference in immunoassays. Strong relationships between the laboratory and the clinician allow communication of this vital information to allow clinicians to more appropriately review their patients results.