Wednesday, September 17, 2008

Specimen Adequacy in Bethesda System

The 2001 Bethesda System requires that every cervical cytology specimen be assessed with respect to its adequacy (Solomon et al., 2002). Specimens are classified into one of two categories: ‘satisfactory for evaluation’ or ‘unsatisfactory for evaluation’. This represents a departure from the 1991 Bethesda System, which also included a third category for specimen adequacy that was called ‘satisfactory for evaluation but limited by…..’ or SBLB. This ‘satisfactory but limited by…’ category was most frequently used when a specimen lacked either endocervical cells or squamous metaplasia from the transformation zone but was in all other aspects ‘satisfactory’.

With the 2001 Bethesda System, cytology specimens previously classified as ‘SBLB’ are classified as ‘satisfactory for evaluation’ and a quality indicator comment is made indicating what limiting features are present. A ‘satisfactory for evaluation’ specimen must be appropriately labelled. To ensure proper identification, the woman’s name or identifying number should be written on, or affixed to, the slide before it is sent to the cytology laboratory. Cytology laboratories should not accept unlabelled slides and should return them to the submitting clinician. It is also critical that the smear-taker provide pertinent clinical information to the laboratory that will evaluate the specimen, including the woman’s age, date of last menstrual period, previous history of abnormal cervical cytology specimens or treatment for cervical disease, and the source of the specimen (e.g., vaginal or cervix).

The minimal cellular requirements for a specimen to be considered ‘satisfactory for evaluation’ in the 2001 Bethesda System vary depending on whether the specimen is a conventional cytology specimen or a liquid-based cytology specimen. For classification of conventional cytology specimens as ‘satisfactory for evaluation’, an estimated 8000 to 12 000 well visualized squamous cells need to be present. For liquid-based cytology specimens, an estimated 5000 cells need to be present. (Figure 29).

Although the selection of these cut-offs is fairly arbitrary, the limit of 5000 cells for a liquid-based cytology specimen to be classified as ‘satisfactory for evaluation’ is based on a cellcounting study in which referent samples were diluted to produce preparations with defined numbers of squamous cells (Studeman et al., 2003).

A clear demarcation in sensitivity was observed using the SurePath™ procedure (see below) between specimens with less than 5000 squamous cells and those with 5000 cells or more: the sensitivity for a reference diagnosis case of low-grade squamous intraepithelial lesion (LSIL) increased from 73% for specimens with less than 5000 squamous cells to 98% for preparations with over 5000 cells. There is much controversy over the importance of identifying a transformation zone component (e.g., squamous metaplastic cells) or endocervical cells in a cervical cytology preparation. Because the majority of high-grade precursor lesions arise within the transformation zone, it was widely believed until recently that specimens lacking a transformation zone component (TZC) or endocervical cells (EC) should be considered somewhat less than ‘satisfactory for evaluation’. This view is supported by several studies that have shown the prevalence of SIL to be higher among cytology specimens that contain TZC/EC than among those that do not (Vooijs et al., 1985; Mitchell & Medley, 1992; Szarewski et al., 1993; Mintzer et al., 1999). However, other studies have failed to confirm this association and, perhaps more importantly, several retrospective longitudinal cohort studies have found that women lacking TZC/EC are no more likely on follow up to be diagnosed with squamous lesions than are women whose specimens contain TZC/EC (Mitchell & Medley, 1991; Mitchell, 2001).

One retrospective case–control study of true positive and false negative cervical cytology specimens from women with CIN 3 found no difference in true positive rates between cases with or without TZC/EC (O’Sullivan et al., 1998). A prospective study of women with normal cytology at entry found that although specimens containing EC at the subsequent test were at significantly higher risk of both low- and high-grade squamous intraepithelial lesions than those without EC, the presence or absence of EC at entry had no significant effect (Mitchell, 2001). In another compelling study on the lack of importance of EC, all negative cervical cytology specimens obtained in the Netherlands between 1990 and 1991 were matched with results of subsequent cytological and histological examinations (Bos et al., 2001). There was no significant difference in the number of women subsequently diagnosed with CIN between women whose initial cytology specimens contained EC and those that did not.

Moreover, the proportions of women diagnosed with cervical cancer were the same in both groups. It is also important to recognize that EC are less frequently found in cervical cytology specimens from women using oral contraceptives, who are pregnant or who are postmenopausal (Davey et al., 2002). It has therefore been argued that specimens lacking EC or a TZC should not be considered unsatisfactory and may not need to be repeated (Davey et al., 2002; Birdsong, 2001; Bos et al., 2001). The 2001 Bethesda System recommends that reports should state whether or not EC or a TZC are present. Specimens lacking endocervical cells or squamous metaplastic cells should be classified as ‘satisfactory for evaluation’ and the quality indicator comment should indicate that these components are not present.

The numeric criterion for stating that such a component is present is 10 well preserved endocervical or squamous metaplastic cells. Specimens in which inflammation, blood or poor preservation cause 50–75% of the epithelial cells to be obscured should be classified as ‘satisfactory for evaluation’, but a quality indicator comment made indicating that there are partially obscuring factors. A specimen is classified as ‘unsatisfactory for evaluation’ when either the minimal number of epithelial cells required for interpretation is not present or blood, inflammation or poor preservation obscures more than 75% of the epithelial cell component(Figure 30).Cases which the laboratory cannot process, such as those received unlabelled, are also classified as ‘unsatisfactory for evaluation’ and no interpretation is rendered.

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