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| Journal of the New Zealand Medical Association,
30-October-2009, Vol 122 No 1305
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Monitoring the performance of New Zealand’s
National Cervical Screening Programme through data linkage
Hazel Lewis, Li-Chia Yeh, Bobby Almendral, Harold Neal
The introduction of an organised cervical screening
programme in New Zealand in 1991 has had a major impact on the incidence of
cervical cancer.1 However the effectiveness of
cervical screening depends on regular participation in the National Cervical
Screening Programme by all women between the ages of 20 and 70 years who have
ever been sexually active, as well as robust quality assurance systems for each
of the Programme’s components (health promotion, smear taking, colposcopy,
treatment, and laboratory services).
The New Zealand Cervical Cancer
Audit2, published in November 2004, was carried
out following the Ministerial Inquiry into the Under-reporting of Cervical Smear
Abnormalities in the Gisborne Region.3 One of
the recommendations following the Gisborne Inquiry was that the National
Cervical Screening Programme (NCSP) should implement a process for ongoing
review of the screening histories of women who develop cervical cancer. Such
case reviews are conducted by several countries with organised cervical
screening programmes,4–10 and should be
distinguished from periodic full scale programme audits.
Evaluation of the performance of the NCSP currently involves
independent monitoring of a range of performance indicators against agreed
targets; regular independent audits of specific Programme components; three
yearly reviews of the Programme as a whole in accordance with the Health
(National Cervical Screening Programme) Amendment Act 2004; ongoing monitoring
of smear takers, laboratories and colposcopy services against the
Programme’s own quality standards; investigation of complaints; and
monitoring trends in Programme outcomes—cervical cancer incidence and
mortality.11
Ongoing review of the screening histories of women diagnosed
with cervical cancer provides a complementary approach to auditing the overall
performance of the NCSP. It could also potentially lead to the identification of
specific problems requiring more detailed investigation and possible corrective
action. The Health (National Cervical Screening Programme) Amendment Act 2004
(s112T–s112Z) provides the legal basis for these reviews.
Women enrolled in the NCSP have their smear histories,
laboratory cytology and histology (biopsy) results recorded in a centralised
database, the National Cervical Screening Programme Register (NCSP-R). All
tissue diagnoses of cancer (including cervical cancers by stage and histological
type) are notified to the New Zealand Cancer Registry (NZCR) under the Cancer
Registry Act 1993.12 Additionally, both
databases identify individuals by their National Health Index (NHI), a unique
personal identifier assigned to all persons at first contact with the health
system.
It is thus possible to link a woman’s cervical cancer
diagnosis (recorded on the NZCR) to her smear history (recorded on the NCSP-R)
via her NHI. Since 2000, the NHI completion rate has been very high (close to
100% on both databases). While there have been problems experienced earlier
(prior to 1997) of multiple NHIs being assigned to the same woman, this has been
greatly reduced due to more active detection and resolution of duplicates on the
NHI database.
Ethical concerns regarding privacy and confidentiality have
to be considered in undertaking any record linkage process in addition to
technical challenges. These concerns were dealt with by special legislation, the
Health (National Cervical Screening Programme) Amendment Act 2004. This provides
the legal basis for the record linkage and ensures that the linked dataset is
maintained securely and is queried only by authorized NCSP staff.
The objectives of this paper are therefore to:
The monitoring process will
eventually include both examination of screening histories obtained from the
NCSP-R (Phase 1) and review of clinical records as well as rereading of cervical
smears and biopsy histologies, where this is indicated (Phase 2). This paper
covers only Phase 1 of the case review process—i.e. the examination of
linked cancer registrations and screening histories.
MethodData sources and matching—Every
quarter, notifications of cervical cancer on the NZCR were matched against the
NCSP-R, with a 6-month delay to allow for processing of notifications by the
NZCR. The period covered for this review was 4 years, from 1 January 2003 to 31
December 2006.
For each notification of cervical cancer, data
extracted from the NZCR included the NHI; demographics (date of birth, domicile,
ethnicity and date of death, if relevant); cancer information including date of
registration, histological type, morphology, and stage.
Staging involved two classifications: histological
staging of cancers as microinvasive versus invasive; and clinical staging using
FIGO (International Federation of Gynecology and Obstetrics).
Each cancer notification was linked to its
corresponding NCSP-R record by NHI. Data extracted from the matching NCSP-R
record included demographics (as above, except ethnicity—NHI ethnicity
used in preference), smear history and histology results (if any). Smear history
included the dates of all smears and the cytology result for each. Histology
results (and dates recorded) from biopsies taken at colposcopy were extracted
and follow-up noted. Both smear histories and histology results were extracted
for the full period for which each woman was enrolled on the NCSP-R—i.e.
for a maximum of 16 years.
Data analysis—For each quarter
that the data were analysed, the same flow chart was used (Figure 1).
Figure 1 Data analysis summary
flowchart
 Firstly, nonsquamous cancers were excluded (i.e.
adenocarcinomas and other histological types of cervical cancer), as cervical
screening has limited effectiveness in preventing these cancers. Therefore,
analysis was restricted to squamous and adenosquamous cancers only.
Secondly, women aged >80 years at diagnosis were
excluded as they were too old to have benefited from the Programme (which began
in 1991 and screens only women <70 years).
Thirdly, ‘eligible’ women (i.e. women
diagnosed at age <80 years with squamous or adenosquamous cervical cancer)
were classified as ‘enrolled’ or ‘not enrolled’ in the
NCSP based on their smear history. ‘Enrolment’ was defined as having
one or more smears recorded 6 months or more prior to the date of diagnosis or
notification of cervical cancer. (A smear taken within 6 months of diagnosis was
considered to be part of the diagnostic process i.e. the cancer was already
present).2
Finally, eligible enrolled women were classified as
‘irregularly’ or ‘regularly’ screened, again based on
their smear history. To be classified as ‘regularly screened’ a
woman must have undergone her first smear before a specified age and then have
had at least one smear every 5 years thereafter to the date of cancer diagnosis
or notification.
Five rather than 3 years was selected to allow a degree
of flexibility in screening. This is consistent with many international
programmes and the natural progression of cervical pre-cancer to microinvasive
and finally fully invasive disease.14 The
specified age was determined by the birthdate of the woman in relation to the
date of commencement of the Programme [1990] (i.e. specified age <30 years if
born >1960, <40 years if born 1950-59, <50 years if born 1940-49,
<60 years if born 1930–39). A woman not meeting these criteria was
considered to have had only irregular or insufficient smears in her
lifetime.
The data were summarised by period (quarter or year),
demographics (age group, ethnic group), cancer type (squamous, adenosquamous),
histological stage (microinvasive, invasive), and smear history (enrolment
status, regularity of screening if enrolled).
Comparison of case reviews with audit
results—Key results from the case reviews for the period
2003–2006 were compared, where possible, with results extracted from the
report of the New Zealand Cervical Cancer Audit (published in 2004), covering
the period 2000–2002.
Differences in methods between the case reviews and
NZCCA should be noted, including differences in the period covered, review of
women’s full medical records and independent reread of slides (both of
which were included in the NZCCA but not in phase 1 of the case reviews).
ResultsFrom 1 January 2003 to 31 December 2006 a total of 625 cases
of cervical cancer were notified to the NZCR. Of these, 430 (69.0%) were
squamous cancers, 31 (5.0%) were adenosquamous, 116 (18.6%) were adenocarcinoma
and 47 (7.5%) were a variety of other histological types.
Of the 461 squamous plus adenosquamous cancers, 23 were
diagnosed in women aged >80 years, leaving 438 cases eligible for linkage.
These comprised 348 invasive and 90 microinvasive cancers. FIGO stage (a
clinical staging system that is distinct from the histological staging of
cancers as ‘invasive’ or ‘microinvasive’) was missing
for 199 (45%) cancers.
For 225 cases (48% of all eligible cases) a definite linkage
was made to a screening record in the NCSP-R (i.e. identical NHI plus consistent
demographic details in both records). These cases were therefore considered to
have occurred in enrolled women.
For a further 202 cases (43.8% of all eligible cases) there
was no matching record on the NCSP-R, or the only smear history recorded related
to a period <6 months prior to the date of cancer diagnosis or notification.
These cases were therefore considered to have occurred in women who had never
enrolled (participated) in the Programme. For the remaining 11 cases (2.4% of
all eligible cases), the possibility that the woman might have previously been
enrolled but had subsequently chosen to withdraw from the Register, or that an
error had occurred in recording either the NHI or the demographic details in one
or both of the two databases, could not be excluded. These 11 women were not
able to be assigned an enrolment status.
Analysis of the screening histories of the 225 women who
were classified as ‘eligible enrolled’ women (i.e. had been
diagnosed at <80 years of age with either squamous or adenosquamous cervical
cancer and were matched to a record on the NCSP-R extending back more than 6
months prior to cancer diagnosis) found that 137 (60.9% of the 225) had only
been irregularly or insufficiently screened; 85 (37.8% of the 225) had been
diagnosed with cervical cancer despite being regularly screened; while for 3
women (1.3% of the 225) no assessment could be made because recording of the
screening history was inadequate. Therefore 31.3% of ‘eligible’
cancers (137/438) occurred in women who were enrolled in the Programme but had
been smeared insufficiently often, or too irregularly, to have derived full
benefit from screening.
In total therefore, 73.5% of eligible cancers (137 enrolled
and 202 not enrolled) (339/438) occurred in women who were unscreened or
insufficiently screened. However, 18.4% (85/438) of eligible cancers did occur
in women who had experienced regular screening (5 yearly or more frequently,
beginning at specified age depending on birth date in relation to NCSP
commencement date).
These results are summarised in Table 1. This table also
shows the distribution of squamous cancers by histological stage (microinvasive
or invasive).
Table 1. Number of cervical cancer cases,
2003–2006
Table 2. Contribution of likely
causes
* Annual average number of eligible squamous and
adenosquamous cases, 2003–2006.
Table 2 shows that, of 110 eligible cases occurring per year
on average, almost half (49%) occur in women who are not enrolled in the
Programme and so are likely to have never been screened. Just under one-third
(31%) occur in women who are enrolled in the Programme but have been screened
only infrequently and/or irregularly. Therefore, combining these two categories,
exactly 80% of potentially preventable cases are estimated to probably result
from inadequate coverage.
The remaining 20% of cases (85 over 4 years or 21 per year
on average) occur despite regular screening (Table 3). These cases represent a
mix of true interval cancers (cancers that develop within 3 years of a confirmed
negative smear) and Programme quality problems (which could have involved any or
all of smear taker, laboratory or colposcopy/treatment issues).
Table 3. Characteristics of eligible women who
developed squamous cervical cancer between 2003 and 2006, despite regular
screening
Note: Number in brackets is percent of
total.
Table 3 shows that cancers among regularly screened women
are more likely than cancers among all eligible women to occur at young adult
ages (27% <30 years compared with 11 %, p=<0.001) Regularly screened women
who develop cervical cancer are also more likely to be diagnosed at a
microinvasive stage than all women eligible for screening (31% microinvasive
compared with 21%, p=<0.001).
Comparison with audit—Table 4
compares key findings from the case reviews for 2003-06 with those reported by
the NZ Cervical Cancer Audit (NZCCA) for 2000–2002.
The results from the two studies show strong agreement,
despite differences in the period included and in methodology. In both the NZCCA
and case reviews, approximately 50% of cancers occurred in unscreened, 30% in
irregularly screened and 20% in screened women. There were some differences in
type distribution, with a higher proportion of squamous and a lower proportion
of adenocarcinomas in the NZCCA. The NZCCA found FIGO stage missing in only
approximately 26% of cases, compared with approximately 50% in the case reviews.
The most likely explanation is that the NZCCA obtained staging data directly
from the records of women with cervical cancer.
Table 4. Comparison of case review
2003–2006 with Audit 2000–2002
Smear histories of women diagnosed at <25 years
of age—The findings of the case reviews of this age group are of
particular interest. These young women will have had only a few smears, so will
be more dependent than older women on the quality of each smear. Cancers in
young women may be more aggressive and fast growing,
27 so interval cancers are more likely in this
age group. Trends in this age group will influence whether the age of screening
initiation can be shifted to age 25 (or even 30 years), as recommended by WHO
and several European countries.14
There were 10 women, (approximately 3 per year over the 4
years studied) with squamous or adenosquamous carcinoma, who were aged under 25
years at diagnosis. Of these 10 cancers, 4 were diagnosed as microinvasive, 4 as
invasive, and 2 had missing histological stage.
DiscussionThe purpose of this paper is to report on one of several
quality assurance processes used by the NCSP to monitor the Programme’s
performance.
This paper demonstrates that the NHI (the National Health
Index, a unique personal identifier) can be used to successfully link cervical
cancer registrations to women’s cervical screening histories as recorded
on the NCSP Register. The linked dataset enables further investigation of the
‘cause’ of cervical cancer to be carried out by an analysis of the
smear histories of women, with relatively little delay (6 months post
registration). Despite differences in method, it was also possible to compare
trends in cervical cancer ‘causation’ with those found in a slightly
earlier time period, through the NZCCA, with highly consistent results.
A key finding from the first 4 years of ongoing screening
history reviews (and from the Audit) is that 80% of potentially preventable
cervical cancers involve women who are not enrolled in the Programme or who have
been only infrequently and irregularly screened. This confirms that improving
coverage remains a priority for the Programme, which was also the main
conclusion of the NZCCA.
It is disappointing that the proportion of cancers
attributable to insufficient coverage has not declined since the NZCCA. However,
a recent OECD report13 suggests that improving
coverage may not be an easy task: at about 72% overall, New Zealand’s
coverage is already among the highest achieved anywhere in the developed world,
being only slightly lower than that of some Nordic countries. However, coverage
is lower for Māori, Pacific, and Asian women, so scope for improvement in
coverage still exists.
At the same time, the review of women’s screening
histories has revealed that each year an average of 21 women will be diagnosed
with this potentially preventable cancer despite having participated fully in
the screening programme. The young age distribution of these cancers and their
higher likelihood of diagnosis at a microinvasive stage (seen in Table 3)
suggest that many could be interval cancers. These women clearly need further,
more detailed investigation to determine which are in fact interval cancers and
which result from quality problems in the screening pathway.
The next phase (Phase 2) will involve the clinical review of
the patient records of these regularly screened women, including re-reading
relevant smears and histology specimens as appropriate. This will need to be
done sensitively and securely, so as not to compromise the privacy of these
women and to minimise the impact of the review on confidentiality of their
personal information. The main objective is to distinguish probable interval
cancers from possible ‘errors’.
It must be emphasised that the objective is not to
‘blame’ providers for isolated errors, but to try and identify
systemic problems that the Programme can address. No screening programme can
ever be 100% effective and a low error rate is inevitable. For some women, no
satisfactory explanation for their cancer may ever be found. Phase 2 will also
provide an independent check of a number of other quality assurance processes
already in place, including the routine ’42-month look back’ carried
out by laboratories (in which the slides of every woman diagnosed with cervical
cancer are re-read)
The review of screening histories may ultimately be extended
to a case control design. This could be achieved by recruiting women enrolled on
the Programme who have not developed cancer to act as ‘controls’.
Comparison of their screening histories with those of the ‘case’
women will strengthen the conclusions that can be drawn as to the reasons why
women continue to develop cervical cancer.
This case review process could also be strengthened in other
ways. Most importantly, the high rate of missing FIGO stage data recorded on the
NZCR needs to be addressed. Documentation of periods when women may have
withdrawn from the Programme (e.g. because of ‘opting off’ or being
overseas) is already being improved through the NCSP-R redevelopment.
Finally, this process of reviewing screening histories will
also contribute to the evaluation of screening policy regarding the appropriate
age to start and stop screening and the screening interval. Of particular
relevance is the question of whether to offer screening to women less than 25
years of age.
Our results show that approximately three cases per year in
this age group are to be expected. Whether this justifies screening women under
25 years of age is a matter of debate. A review of the literature shows that, as
in our study, the main reason for failure to prevent cervical cancer is lack of
participation in regular
screening.4–10,15–22
A recent study of 225 women with cervical cancer in
Canada16 found that 68% of women who developed
cervical cancer had not participated in routine screening and 15% had
participated only sporadically. Another study of women with cervical cancer in
Denmark found that 23% had never had a smear and 61% did not participate
regularly in screening.15
However, it is also well recognised that the conventional
Pap smear has limitations as a screening test and has a false negative rate of
about 20% for high grade lesions.2,26 Yet in
spite of these limitations, the NCSP has achieved a 50% reduction in incidence
and 60% reduction in mortality since becoming
operational.1
In order to encourage women to participate in cervical
screening the National Screening Unit launched a communications campaign in
September 2007, targeting Maori and Pacific women. This has resulted in a slow
but steady increase in participation in the
Programme.28 The Programme has also recently
reviewed the 1999 guidelines for the management of women with abnormal smears as
well as new technologies (such as HPV testing) to improve overall Programme
performance. New ways of managing cervical abnormalities in conjunction with new
technologies are likely to be needed as we enter the era of HPV
vaccination.23-25
In conclusion, our experience over 4 years has demonstrated
that ongoing linkage of cancer data to screening data provides a robust approach
to monitoring the performance of the NCSP alongside other approaches. This
process should continue but needs to be accompanied by a more detailed
investigation of the small number of women who develop cancer despite regular
screening (Phase 2 investigation). Recruitment of control women should also be
included, so that causal conclusions can be more reliably drawn through use of a
case control study design.
Note: This paper is published in
accordance with The Health (National Cervical Screening Programme) Amendment Act
2004 (section 112S).
Competing interests: None known.
Author information: Hazel Lewis, Clinical
Leader, National Cervical Screening Programme, National Screening Unit, Ministry
of Health, Wellington; Li-Chia Yeh, Statistician, Public Health Intelligence,
Ministry of Health, Wellington; Bobby Almendral, Technical Specialist, National
Screening Unit, Ministry of Health, Wellington; Harold Neal, Scientific Advisor,
National Cervical Screening Programme, National Screening Unit, Ministry of
Health, Christchurch
Acknowledgements: We acknowledge assistance
of Jane McEntee (previous NCSP manager), Diane Casey (current NCSP manager), and
Lesley Mack, Pauline Fallon, Jennifer Beaulac (previous NCSP policy analysts).
We are grateful to NZHIS staff for data matching; including
Tracey Vandenberg, Ian Galley, Rebecca Kay, and Rebecca Hislop.
We also acknowledge peer review by Alistair Woodward and
Lynn Sadler (University of Auckland); Philip Castle (NIH, USA); and Eileen Hewer
(National Screening Unit) who reviewed this article.
We thank Michelle Hooper for administrative support.
Correspondence: Dr Hazel Lewis, Clinical
Leader, National Cervical Screening Programme, National Screening Unit, Ministry
of Health, P O Box 5013, Wellington, New Zealand. Phone 04 816 4343, Fax 04 816
4484, email: hazel_lewis@moh.govt.nz
References:
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