Our data clearly show the exceedingly low risk of CIN3 or cancer following a negative HPV test result, in follow-up lasting up to 15+ years. These data corroborate and extend those of other long-term cohorts (5
) and our own previous publications on the Portland cohort (14
). Cervical cancer almost always arises from detectable persistence of HPV infections (10
). A negative test provides excellent “negative predictive value”, i.e., confidence that the “clock is still set at zero” in the typically decades-long natural history of cervical cancer. The risk following a negative test is increasingly low as women age (5
) and become less likely to acquire new infections, which, even if acquired, would tend to clear as in younger women (25
Except in low-resource regions, cervical cancer screening is a repeated process. Still, as recently suggested by the authors of several cohort studies and randomized clinical trials (4
), the strong long-term reassurance from a single negative HPV test should inform clinical practice. We are not suggesting that screening in high-resource regions be extended to the length of follow-up of the Portland Cohort. However, re-screening at three years for women 30 and older after negative HPV and cytology co-testing, as commonly recommended (11
), is possibly too soon; it is concerning that many clinicians are repeating HPV tests at even shorter intervals (28
). Even at older ages, HPV detected within three years after a negative test, like any new infection, will rarely progress to >CIN3 let alone cancer (25
Accumulated evidence and conclusions reported recently from Europe (4
) support longer intervals of five to seven years are being considered. In a pooled study of approximately 25,000 women enrolled in seven European cohorts, the cumulative incidence rate of ≥CIN3 after a negative HPV test at baseline was 0.3% (95% CI of 0.1% to 0.4%). This is only slightly lower than the cumulative percentage of 0.7% (95% CI of 0.5% to 0.9%) that we observed in HPV-negative women 30 and older.
Our results might be most important for low-resource regions where frequent screening is not the norm. The low risk following HPV-negative screening frees health planners in low-resource regions from a perceived ethical requirement to emulate the Western model of frequently repeated cytology screening and justify the use of HPV testing at extended intervals using new low-cost HPV tests (29
Compared with other carcinogenic types, higher risk of ≥CIN3 remained for a decade after an HPV16-positive test (5
). Despite the large study population, we had less power to study less prevalent carcinogenic HPV although our data did (without statistical significance) corroborate elevated risk following infection with HPV18 or HPV31(24
), and little elevation in risk for other carcinogenic types (5
). As shown by other cohorts, the CIN3 lesions detected by HPV testing especially in women 30 and older are often (but not always) persistent and therefore clinically significant (5
). We need health-decision analyses to determine if distinguishing HPV16 or other especially carcinogenic types in HPV test kits is cost-effective, and how we might manage different levels of risk once accurate typing is widely available clinically. In this study, we found no support for separate typing for the majority of the carcinogenic types although numbers of cases caused by most types were limited.
The comparison of HPV testing with cytology indicated that HPV status provides longer and stronger risk stratification than cytology, supporting the conclusions of previous short- and long-term comparisons (4
). Finding cytologic ASC-US/LSIL added little long-term risk stratification to that provided by HPV testing. In the European pooled analysis (4
) after six years the risk of CIN3 among initially HPV-negative women was lower than after three years among cytology-negative women, suggesting again that switching to HPV testing should result in extended screening intervals. In making this comparison, it is important to note that cytology was not designed for single-time, long-term use but rather as part of a screening routine to be repeated at fixed intervals. We also note that are specific comparison was limited by the non-specificity, particularly among older women, of equivocal cytologic interpretations that we classified as ASC-US. With more contemporary cytology, many would now be called negative. As another limitation, some of the early elevated risk of diagnosis of ≥CIN3 in our study was due to diagnostic efforts triggered by cytologic abnormalities.
As more minor points, we found that the importance of HPV16 was attenuated for CIN2, which can be caused in larger part by many HPV types and which, even more than early CIN3, frequently regresses spontaneously (5
). We also confirmed at enrollment that most HPV infections, especially single infections at older ages, do not cause concurrent cytologic abnormalities (35
Study limitations influenced the results, if not the conclusions. We collected cells with a cervicovaginal lavage, which proved insensitive among older women. The lack of sensitivity of the cervical lavage collection method among older women might reflect that their cervical transformation zones were more likely to be within the cervical os (29
), even though the median age of women with ≥CIN3 in the older age group was only 38 and HPV-negative case women were not significantly older than HPV-positive ones. It could also reflect the higher prevalence of HPV infections in the younger age group.
This lowered sensitivity led to decreased negative predictive value among older women. There is a possibility that this decrease could be greater for particular HPV types more prone to infect the cervix as opposed to the vagina (36
We tested specimens multiple times, until some had no available satisfactory aliquots. Thus, we had to depend on a combination of the test results, which is a major difference from routine clinical practice and a key limitation of this long-term cohort project. Most specimens negative by HCT and HC2, in women never diagnosed with any abnormality, were never tested by PCR. A small percentage of such specimens (as seen in the random subcohort, data not shown) were PCR positive; in absolute numbers, these missed infections could have accounted for a sizable minority of all infections in the cohort. Therefore, because they never led to any cytologic or histologic abnormal diagnosis, the PPV estimates we presented might be somewhat elevated; a more sensitive testing scheme would perhaps lower the risk (i.e., the PPV or the cumulative probability) after a positive test, but strengthen the reassurance from a negative test.
Women, especially younger ones, tended to leave the Kaiser health plan over the years, leading to small numbers and unstable estimates for some analyses. Reassuringly, our results corroborate the recent report of a cohort study among women in Copenhagen younger than 30 years at enrollment (24
We had too few events in the low risk, highly screened Kaiser cohort to assess the carcinogenicity of HPV types other than HPV16. Despite the large size of the Portland Cohort, reduced numbers of women infected with any specific type returned after 10 years. With limited precision, we did observe expected elevated cumulative risks of ≥CIN3 for HPV31 and HPV18. The risk of invasive cancer in this cohort was clearly linked to only a few HPV types.
Finally, we did not have repeat, longitudinal HPV testing of the kind that have shown that is HPV persistence that leads to CIN3 and cancer (24
). In particular, we did not have genotyping of the lesional tissue at the time of diagnosis preventing exact attribution of each case to a specific HPV type (37
). It is important to realize that diagnosis of ≥CIN3 following detection of an HPV infection does not prove causality, especially during long-term follow-up. Even when a single HPV type is present at enrollment, our “single snapshot” study cannot rule out that the detected type cleared, and another subsequently acquired type eventually progressed to precancer. We believe that the continuing accumulation of CIN2 especially among younger women indeed suggests that some endpoints in our study arose, perhaps very quickly, from infections acquired subsequent to specimen collection and are not related to enrollment infections. If so, the long-term low rates of >CIN3 following a negative HPV test are even more remarkable. Although we cannot be absolutely certain of the causal HPV genotype for follow-up >CIN3 based on the baseline measurement of HPV, we note the strong predictive value of HPV16 and that roughly half of all >CIN3 as expected were HPV16 positive at baseline. Therefore, we surmise that for the vast majority of cases found during follow-up, the causal HPV genotype was present at baseline.
In conclusion, long-term data from women spanning all adult ages in the Portland Cohort Study strongly support the negative predictive value of HPV testing. A double negative result of HPV-negative NILM predicts an extremely low risk of ≥CIN2 in the subsequent decade. The risk decreases even further as women age from 30-39 to 40+. Logically, screening intervals should widen further as successive double negative screenings are accrued. A remaining challenge is to determine how to manage HPV positive women, particularly those with HPV16 or other especially high-risk types, if a switch to HPV testing for primary screening includes HPV typing.