Introduction

GLOBOCAN and the Global Burden of Disease (GBD) estimates place Mexico’s cancer mortality among the lowest in the Americas. GBD‘s 2017 age-standardized cancer mortality rate for Mexico was 86.3 per 100 000, the lowest in Latin America only after Nicaragua (71.3 per 100 000). ^1,2Mexico’s low cancer mortality is paradoxical, given the ageing of the population, the epidemiologic transition, and the high frequency of cancers detected in late stages. ^3,4,5

Understanding this paradoxical observation is important to strengthen health information systems, accurately characterize the burden of disease, and guide etiologic research. We hypothesized that Mexico’s low cancer mortality could be attributable to pitfalls in death certificate coding and attribution of underlying cause of death in national mortality registry used for mortality statistics.⁶We aim to explore death registration inaccuracy by comparing cancer mortality rates from Mexico’s official death registry with a death registry from a disease surveillance system that independently processes death certificates.

Materials and methods

Results

Between 2010 and 2014, there were 366 958 cancer deaths from all sites according to INEGI and 364 618 according to SEED (<1% difference). We observed minimal differences in age-standardized mortality rates for cancer and site-specific cancer mortality for either sex (table I). Cancer mortality from all sites in women per 100 000 was 73.3 (95%CI 73.0, 73.6) for INEGI and 72.7 (95%CI 72.4, 73.0) for SEED. The corresponding estimates for men were 68.3 (95%CI 67.9, 68.6) and 67.8 (95%CI 67.5, 68.2). Rates were similar even for site-specific neoplasms. For breast cancer, the mortality rate per 100 000 women was 10.4 (95%CI 10.3, 10.5) for INEGI, while SEED reported 10.5 (95%CI 10.3, 10.6). INEGI reported 10.6 (95%CI 10.4, 10.7) per 100 000 men for prostate cancer, while 10.5 (95%CI 10.4, 10.6) was recorded by SEED. We found considerable differences in mortality estimates for stroke, MI, and CKD when comparing INEGI to SEED. Differences were particularly striking for CKD (females: 8.5 vs. 6.8 per 100 000 for women, and males: 10.2 vs. 8.3 per 100 000 for men).

Garbage codes for cancer deaths were practically the same for both registries (table II). SEED had slightly more garbage codes compared to INEGI for deaths from cardiovascular disease. When we explored the impact of including as cancer deaths those with a cancer diagnosis in the contributing causes of death, the number of deaths increased by 2 431 for INEGI and 1 474 for SEED but cancer mortality rates were minimally modified. For INEGI, cancer mortality per 100 000 in women increased from 74.8 to 77.4 and from 69.0 to 71.3 in men after inclusion of these potential cancer deaths. The corresponding increases in mortality rates for SEED were 74.1 to 75.5 per 100 000 in women and 68.4 to 70.0 per 100 000 in men.

Discussion

Cancer mortality estimates were essentially equal, based on two independently processed mortality registries. The percentage of garbage codes for cancer was very similar and cancer mortality estimates were not affected after inclusion of cancer diagnoses that were not considered the underlying cause of death.

Our results suggest that Mexico’s low cancer mortality is unlikely to be explained by death certificate processing. In Mexico, agreement between death certificates and medical records appears to be moderately high for neoplasia (85% agreement), but not for hypertensive diseases, diabetes, and infections.⁶ The 25% difference in mortality estimates for CKD underscores the challenges of assigning causes of death and warrants a more detailed investigation of the potential source of this difference given the increasing importance of the burden of CKD in Mexico. ^11,12This finding also sheds light on the fact that although Mexico’s official death registration has been rated with the highest quality based on completeness and coding characteristics, inaccuracy in data processing may be present for certain diseases. ¹³ More research is needed to evaluate the potential inaccuracy of death certification according to geographic area, type of health facility where the death occurred, and personnel who completed the death certificate. While INEGI’s database remains the gold standard for national statistics, researchers must be cautious when choosing which data source to use for research.

The most straightforward explanation for Mexico’s low cancer mortality would be low cancer incidence. However, more likely possibilities include cancer underdiagnosis (as a result of low cancer screening coverage or lack of availability of diagnostic tools), limitations in access to cancer care, competing causes of mortality due to increasing incidence of diabetes, and infrequent necropsies.^14,15,16 Future studies are needed to evaluate the quality of medical diagnosis at death using necropsies, especially in rural and public/at home deaths. Mexico has only recently established population-based cancer registries. These registries will provide insights on cancer mortality estimates and critical information on cancer prevalence, incidence, and survival.

This study is not without limitations. First, in order to fully understand the nature of the discrepancies, a one-on-one record comparison would be necessary to discern error derived from inaccuracies in coding deaths from errors in adjudication of the underlying cause of death. However, current data protection policies preclude this possibility. Second, we are unable to assess whether low cancer mortality was due to errors in death certificate recording by clinicians rather than death certificate processing. Finally, most of the deaths reported in both databases occurred in the adult population. It would be interesting to evaluate if the discrepancies shown in this study translate to the pediatric population when evaluated separately.