The clinical laboratory may perform additional test or tests to ascertain the integrity of immunoassay analyses when the results are in doubt. Doubling serial dilutions is one of these confirmatory tests. Doubling dilutions is a useful and informative test for detecting inaccuracy and potential interference from endogenous antibodies in immunoassays. Despite its technical simplicity, the science underpinning this test is highly complex. Appreciating such complexities could help in understanding the various outcomes of such tests from misleadingly inaccurate but virtually linear/parallel results, despite the presence of interfering antibodies in some immunoassays, to an array of non-linearity/non-parallelism synonymous with interference in others. A lack of parallelism/linearity could identify some 60% of samples with inaccurate and potentially misleading results caused by endogenous interfering antibodies. Assessing real-time binding kinetics using techniques such as non-linear regression analysis or linearised transformations such as the Scatchard plot could help in identifying more samples in which the doubling dilutions test remains linear and parallel.