Background and aims Results of the immunofluorescence (IIF) ANA test are commonly reported as positive or negative, and a titre is also reported. The test is not standardised. The value of the IIF ANA test has been questioned because of a large proportion of apparently false positive results. We show here that ANA quantified by estimation of immunofluorescence intensity calibrated against the ANA reference preparation WHO 66/233 is highly predictive of the occurrence of anti-dsDNA and anti-ENA in the same serum, and is therefore a cost efficient screening test for these antibodies.
Methods In a cohort of 85 542 patients tested for ANA for the first time during the 11 years 2000 to 2010, the 50th percentile was 2 IU/ml, the 90th was 10 IU/ml, the 95th was 20 IU/ml and the 99th was 30 IU/ml. The relationship between ANA quantitated in IU/ml with anti dsDNA and anti ENA (histones, RNP, SSB/La, Sm and, Scl70), in terms of percent predictive value and odds ratios are shown in Table 1.
Results The predictive value was higher in younger individuals and females as shown in Table 2.
Conclusions ANA quantitated in IU/ml and taking into account age and gender is an efficient screening test to select which sera require testing for anti dsDNA and anti ENA. It would permit this testing to be safely avoided in some 90% of sera.
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