R. G. Blanks and R. S. Kelly
Comparison of cytology and histology results in English cervical screening laboratories before and after liquid-based cytology conversion: do the data provide evidence for a single category of high-grade dyskaryosis?
Objective: To determine whether the difference between the positive predictive value (PPV) for cervical intraepithelial neoplasia (CIN) grade 2 or worse (CIN2+) of referral from moderate dyskaryosis and from severe dyskaryosis was reduced after laboratories converted from conventional to liquid-based cytology (LBC). Furthermore, to explore the cytology/histology agreement after LBC conversion, and to determine post-LBC whether there was increased support for the use of one single category of high-grade dyskaryosis (equivalent to high-grade squamous intraepithelial lesion).
Methods: The association between cytology and histology has been examined using annual Korner return data (KC61 returns) collected by laboratories from the English National Health Service cervical screening programme. The study compares return data before and after LBC conversion.
Results: The study examined data from 102 laboratories that converted from conventional cytology to LBC. Before conversion the PPV for CIN2+ of severe dyskaryosis was 88% and after increased to 90% (P = 0.003). For moderate dyskaryosis the PPV for CIN2+ increased from 70% to 72% (P = 0.06). The absolute difference of 18% between severe and moderate dyskaryosis was therefore the same pre- and post-LBC conversion. The PPV of mild dyskaryosis for CIN2+ before and after conversion reduced from 23% to 19% (P < 0.001). The agreement between cytology and histology measured using a weighted Kappa statistic increased from 0.52 to 0.60 after conversion to LBC because of small increases in the proportions of severe dyskaryosis or worse with CIN3+ outcomes and mild dyskaryosis with CIN1 or less outcomes.
Conclusions: Following LBC conversion there was evidence of a modest increase in the agreement between cytology and histology but no evidence of a change in the absolute difference in PPV for CIN2+ between moderate and severe dyskaryosis. The data support the conclusion that women referred with moderate dyskaryosis will on average have a lower risk of progression to invasive cancer than women referred with severe dyskaryosis. If the data were considered to support the categories of high-grade dyskaryosis (moderate) and high-grade dyskaryosis (severe) before LBC conversion then it can be strongly argued that they also support these categories after conversion.