Laboratory performance measures: evidence against low-risk women explaining low detection rates of high-grade...

When a laboratory has a low reporting rate of high-grade abnormality for its cervical cytology specimens, the question arises whether this is due to the laboratory screening a low-risk group of women. This study was undertaken to explore the hypothesis that a low-risk group of women were screened by Victorian laboratories not meeting the recommended minimum standard for the detection of high-grade abnormalities.

A cohort of 28 094 women was established comprising women whose cervical cytology was reported in 2000 by laboratories not meeting the recommended standard of 0.5% high-grade abnormalities in their reporting of community smears. Outcome measures included the prevalence of high-grade abnormality on the next cytology test for the women, the positive predictive value of the cytology reports of high-grade abnormality, and the standardized incidence ratio for a later diagnosis of cervical cancer.

The prevalence of high-grade abnormality was 0.36% on the index cytology compared with 0.68% on the first subsequent cytology report. Sixty-nine per cent (60/87) of the index cytology reports of high-grade abnormality were confirmed as high-grade abnormalities on histology, compared with 70.8% (121/171) for the first subsequent reports of high-grade abnormality. During 70 015 person-years at risk, nine cases of cervical cancer were observed compared with 5.21 expected cases, giving a standardized incidence ratio of 1.73 (95% CI 0.79-3.28).

These results do not support a hypothesis that the low detection rate for high-grade abnormalities is due to the women comprising a low-risk group for cervical neoplasia.