Journal of Immunological Methods, 91 (1986) 249-255 249 Elsevier JIM 04006 Antinuclear antibody Precise and accurate quantitation without serial dilution * E. Bonifacio, P.N. Hollingsworth and R.L. Dawkins ** Department of Clinical Immunology, The Queen Elizabeth H Medical Centre, Verdun Street, Nedlands, Western Australia 6009, Australia (Received 15 January 1986, accepted 24 March 1986) A new method for measuring homogeneous pattern antinuclear antibody by immunofluorescence has been validated. This method exploits the differential sensitivity of rat heart muscle, kidney tubules and liver parenchyma as substrates for ANA, and employs calibrating sera. It provides quantitative measure- ment in the range 2.5-10 WHO IU/ml and semi-quantitative measurement up to 30 WHO IU/ml, without the need for serial dilution. The routine use of standards together with this method improves precision and provides conversion of measurements to IU/ml. These modifications make ANA a precise screening test for the exclusion of SLE. Key words: Antinuclear antibody; WHO units; Diagnostic role; Immunofluorescence;Precision; Systemiclupus erythematosus Introduction For more than 20 years the immunofluores- cence test has been used as a means of detecting some antinuclear antibodies (ANA) but it is not always clear how the results are used diagnosti- cally. The standard introduced by WHO does not appear to be used widely and few studies incorpo- rating ANA measurements refer to the precision * Publication Q8523 of the Departments of Clinical Im- munology, Royal Perth Hospital and The Queen Elizabeth II Medical Centre, Perth, Western Australia, Australia. ** To whom correspondence should be addressed. Abbreoiations: ANA, antinuclear antibody; WHO, World Health Organization; SLE, systemic lupus erythematosus; WHO IU, World Health Organization international units; SEAPAL, South East Asian and Pacific Area League against Rheumatism; SU, SEAPAL units In this publication concentration is used to mean units of ANA per volume, rather than in the strict sense of mass per volume. of the test. Proficiency testing in the U.S.A. has revealed differences up to 64-fold in the titre of replicate sera tested by multiple laboratories using various methods (Taylor and Fulford, 1980). The authors were incredulous that physicians could use such test data. An on-going exchange programme in the SEAPAL region has indicated that there are similar problems (Bonifacio et al., 1986). Labora- tories interpret their results in diverse ways and few achieve acceptable precision. Quality assurance programmes are available and may disclose imprecision and/or inaccuracy. However they do not provide remedies. There is a need for simple practical methods of ensuring and controlling end point precision, correcting for be- tween-run variation in assay sensitivity, conver- sion of measurements to WHO international units (IU) per ml and maintaining concordance between laboratories. In this report we describe some simple ap- proaches to quality control and illustrate how their use has led to improvements. In designing 0022-1759/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)