Data quality indices – Implications of lack of active follow up

The fact that 25 out of 28 registries from low or medium resource countries that have undertaken population-based survival studies had contributed data to IARC CI5C publication series (Parkin et al., 2005, Curado et al., 2007) at one time or other, stands testimony to the data quality on cancer incidence. However, in 15 registries, the mortality data were not published as they were not routinely available or were included with considerable reservation. Thus, the important data quality issue in a survival study is achieving adequate follow up to get vital status data whether the patient is alive or dead at the end of the study. In a low or medium resource setting, with demonstrated less-developed mortality registration systems, achieving adequate complete follow up is possible only if registries undertook special efforts by evolving a variety of active methods suiting the conditions. Survival reported by most registries that pursued follow up entirely by active methods, tended to reflect the average outcome from the different cancers studied, keeping with the advanced stages at presentation, standards of health care development in their regions, inequities in accessibility to services especially cancer directed treatment and compliance to it and minimal or no cancer screening facilities. Interestingly, the countries that achieved the highest survival in this study have pursued follow up of cases predominantly by passive means with minimal active components.

Box plots have been employed to examine the relationship, if any, between the estimated 5-year ASRS and four categories of registries classified based on methods (AE, AP, PE and PP according to Table 4) adopted for follow up data collection for vital status. The published five-year age-standardized relative survival (ASRS) percent values for cancers of the breast (Figure 3a) and cervix (Figure 3b) were utilized from registries that contributed data registered during 1990-2001 and period varying for individual registries (Sankaranarayanan and Swaminathan, 2011). The median, quartiles and range of ASRS (0-74

years) values showed a gradual ascendancy from entirely active to entirely passive methods of follow up. This phenomenon was true for most cancers with high lethality as well (Sankaranarayanan and Swaminathan, 2011). This suggests a possible methodological problem of follow up, especially in the ascertainment of deaths, as demonstrated in Chennai registry data, resulting in substantial bias in the actuarial survival estimate under standard assumptions.

Figure 3a: Breast cancer 5-year Age Standardised Relative Survival (ASRS; 0-74 years) by classified methods of follow up in 26 registries, 1990-2001

3 8

3 9

N =

Method of follow up

Passive only Predominant passive Predominantly active

Active only

ASRS (0-74 years) %

100

80

60

40

20

0

Figure 3b: Cervix cancer 5-year Age Standardised Relative Survival (ASRS; 0-74 years) by classified methods of follow up in 23 registries, 1990-2001

High level of completeness of both cancer incidence and ascertainment of mortality data are important prerequisites for valid cancer survival estimates and when such completeness cannot be assured, survival rates and their comparisons should be carefully interpreted. Even modest levels of under-registration of deaths may lead to severe overestimation of long-term cancer survival estimates (Brenner and Hakulinen, 2009).

Mortality ascertainment will be sub-optimal in a passive follow up environment if the data linkages between mortality and incident cancer registry databases are not based on a unique personal or national identification number and not backed by a sound death registration system. Such deficiencies result in incomplete follow up. This is even more accentuated by the fact that death registration is generally done based on place of occurrence of death and not necessarily on usual place of residence. The registries generally have access only to official mortality data of the region covered by the registries. This discounts the possibility of knowledge of deaths of cancer patients occurring outside of the registry coverage area if

the definition of location of death in health services research and death registration system.

Hence, such defective linkages effectively means that the extent of incompleteness in follow-up is unknown, especially mortality and such dead cases would have been erroneously classified as alive at the closing date of follow up. Therefore, the reported survival from some of the registries in the study may clearly be over estimated.

Aside studies have been carried out in the past to estimate the completeness of follow up, especially mortality, in a passive environment by extra active follow up for selected cancers, which revealed missing of deaths through routine linkages (Berrino et al., 1995). The present study has extended the elucidation of the bias in the estimation of absolute survival under different assumptions on vital status of patients depending on purely or predominantly active or passive methods of follow up for major cancers using Chennai registry data from India. This analogy is applied to cover the age-standardized relative survival rates for the same cancers in Table 18.

Table 18: Absolute increase in Age Standardised Relative Survival (0-74 years)% between treating loss to follow up as is and assuming them as alive at closing date in the absence of active follow up in Chennai, India, 1990-1999

% lost to follow up: Years from

The implications of lack of active follow up on population based survival in Chennai, India, under two assumptions on the survival status of patients for major cancers registered during 1990-1999 and followed through 2001 in Chennai, India, are given in Table 18. It shows the 5-year ASRS% values by assuming that all loss to follow up cases in reality, as alive on 31^st December 2001 (Case 2 in methods section 4.3.6; figure 2) and by treating all losses to follow up cases with actuarial assumption (Case 3 in methods section 4.3.6; figure 2). An upward bias ranging between 3-13% under 5-21% of losses to follow up for different cancers was detected. Extending the same analogy to other registries that pursued predominantly passive methods of follow up to get vital status information, if the losses to follow up did not exceed one in five and they were not correlated with survival, the over-estimation of 5-year ASRS would have conformed to the upper limit of 13%. The more the losses, the higher would be the bias. The bias would be much more if the mortality ascertainment by passive follow up was poorer than anticipated. Therefore, it is imperative for registries from low or medium resource countries to evolve suitable methods of active follow up before embarking on survival studies.