The development of the referral outcome diagram and an analysis of laboratory cancer detection rates in the English NHS cervical screening programme--is there an optimum level of detection of CIN 1 and CIN 2 lesions?

OBJECTIVE

To use routine annual data from the English cervical screening laboratories (KC61 returns) to evaluate individual laboratory return characteristics with particular reference to factors associated with sensitivity and specificity.

METHODS

A graphical technique has been developed using data on referral to colposcopy and histological outcomes called a referral outcome (ROUT) diagram. The average grade of cervical intraepithelial neoplasia (CIN) detected (the mean CIN score, MCS) is plotted against the odds of a false-positive referral. Further analysis has been conducted to examine the relationship between the MCS and screen-detected invasive cancer rate.

RESULTS

There are large variations in ROUT diagram positions of individual laboratories and the diagram can be used to identify laboratories for further investigation. These variations are strongly influenced by substantial differences in the rate of low-grade referrals and the MCS (and positive predictive value) are inversely related to the referral rate for low-grade cytology (P < 0.001). There is a strong association between high MCS values and increased screen-detected cancer rates (P < 0.001) particularly above an MCS of 2.2. The data can be re-formulated in terms of CIN 2 and CIN 3 only where it can be shown that the invasive cancer rate rapidly increases if the numbers of CIN 2 lesions detected drops below 50% of the number of CIN 3 lesions. Given the complexity of cervical screening this may best be viewed as a hypothesis generating observation, best tested by interventional studies.

CONCLUSIONS

The ROUT diagram represents a new and potentially interesting way of presenting annual return data. The national programme in England needs to balance the prevention of cancer against too many unnecessary referrals to colposcopy and the ROUT diagram, and associated data given in this paper may help toward this. Further research is required including examining the role of referral policy and threshold criteria in influencing low-grade referrals and the relationship between MCS and cancer detection rate.