Precise and accurate results (by the processes of internal quality control (IQC) and external quality assessment (EQA)) and a timely and appropriate support (by means of a laboratory audit, clinical audit, laboratory accreditation and clinical governance) are generated by the delivery of a quality (defined as a degree of excellence in the Oxford English Dictionary) service in clinical immunology. The major objective of quality assurance is to improve the quality of results for uniformity both within and between laboratories, so that an appropriate clinical interpretation can be made on the basis of that result. basis of that result. Within this objective, the role of IQC is to monitor the day\to\day precision and accuracy of a given assay. The role of EQA is broader in that it can compare and contrast different methods, thus also providing educational information. The control of immunoassay systems can be especially challenging owing to the complexity of the methods used. Variability SMN may be introduced at many levels, including the antigen source (whole tissue, cell extract, purified protein, recombinant protein), antibody detected (isotype, affinity, concentration), antibody detection system (polyclonal, monoclonal, affinity, conjugation (enzyme, fluorochrome) and methodological variations (incubation time, volume, choice of substrate). All these factors are taken into account when things go wrong. This article suggests an approach to IQC, including the mathematical methods used to monitor precision and accuracy, appropriate reference materials and controls, and some internal checks that help in providing additional information. Reference materialsthe starting point for assured quality Both IQC and EQA have underlying requirements for central reference materials (often incorrectly called standards) to enable direct comparisons to be made between laboratories. During the 1960s and 1970s, the World Health Organization UPGL00004 and the International Standards Organisation outlined the requirements for the preparation of references. The major requirement was for a stable homogeneous material to be available in significant volume that would behave in the same way as the appropriate body fluid.1 Several categories were defined according to the type of preparationfor example, whether a numerical value was assigned or not. Table 1?1 lists some of the reference materials more useful to the clinical immunology laboratory. On a day\to\day basis, the laboratory is more likely to use a secondary standard derived from these, either internally or by a UPGL00004 manufacturer. Table 1?Some reference materials for use in clinical immunology immunoprecipitation assay). This often represents a shift in the balance between disease specificity and technological sensitivity. Equally important are the materials used to monitor performance (quality\control materials). Often, the manufacturer of the assay under consideration will provide these materials, of which many are simply dilutions of the same material that is used as the standard. This is far from ideal, because if there is any deterioration in the standard, there will be a parallel deterioration in the control and the changed calibration may not become obvious for a considerable time. It is good practice to UPGL00004 include a control from a separate source (third\party controlthat is, material from another manufacturer), UPGL00004 which, hopefully, will not deteriorate at the same rate. A recent example where this problem occurred was highlighted in the UK National External Quality Assessment Scheme for complement in 2004, where, over a period of about 6?months, it gradually became clear that one manufacturer had a calibration error of about 15% in the C4 secondary standard. It is normal practice to control the assay by using the manufacturer’s controls. Use of a third\party control, separately calibrated, may have identified the calibration error sooner. Important concepts 1. Precision is the ability to obtain the same result on a given analyte each time a given material is analysed. For any given assay, the precision is lower as the level of the analyte approaches the lower level of sensitivity of the assaythat is, an assay is less precise at the bottom end of the measurable range. Mathematically, precision (or more correctly imprecision) is expressed as the coefficient of variation, where coefficient of variation?=?SD/Mean100%. Precision may worsen as a result of deterioration of reagents, instability of the apparatus or owing to UPGL00004 operator problems. Table 2?2 shows suggested levels of acceptable precision . Whether these levels are achievable varies considerably with the method, analyte, operator experience and clinical requirement. For example, ELISAs for anti\cardiolipin antibodies are difficult to reproduce and often have coefficient of variation 25%. Typically, clinically relevant antibody levels are high.2 As a result of this, and to ensure clarity for the user, I suggest laboratories consider reporting only qualitative resultsthat is, negative, weak, moderate or strong positive. This approach has achieved higher interlaboratory concordance than the numeric data in the UK National External Quality Assessment Scheme. These figures are based on the author’s experience and are not necessarily applicable to all assays. Table 2?Suggested acceptable levels of precision ELISA) and to the type of action point (eg, single point outside 3 SD a trend of five rising points.) A one\off erroneous point suggests a sampling error, by either the analyser or the operator. Where a trend is seenthat is, a gradual rise or fall on the Shewhart charta change in accuracy, usually caused by deterioration of reagents or reference material, is indicated. In ELISA, for example, substrate, marker antibody, plate coating.