Use of a standard curve improves precision and concordance of antinuclear antibody measurement.

An exchange of sera between 31 laboratories in Australia and 11 other countries has demonstrated problems in measurement of antinuclear antibody by immunofluorescence. Many laboratories lack precision and there is major discordance between laboratories. Four laboratories appeared unable to measure ANA in the concentration range from 2.5 to 20 IU/ml and only 13 out of 28 could discriminate between 7 IU/ml and 15 IU/ml. Incorporation of common standard sera to generate a standard curve for each assay led to improvements. Use of the standard curve to correct for variation between assays improved precision, and conversion of local titres to common units (IU/ml) improved concordance. This inexpensive and simple strategy should improve routine measurement of ANA and thereby increase its diagnostic value.