CHAPTER 9: Cohort Studies
9.3 Follow-up
Vital status ascertainment In some instances, particularly in community based studies, follow-up may involve regular contact with the study participants, including repeated surveys of health status. Perhaps more commonly, follow-up may not involve further contact with the study
participants, but may be done by routine record linkage.
For example, study participants may be followed over time by linking the study information with national death records, or incidence records (e.g. a national cancer registry) as well as with other record systems (e.g. social security records, drivers license records) to confirm vital status in those who are not found to have died during the follow-up period.
Although most developed countries have complete systems of death registration, and it is easy in theory to identify all deaths in a particular cohort, this may not be so straightforward in practice. For example, many countries do not have national identification numbers and record linkage may have to be done on the basis of name and date of birth. This may not be infallible because of
differences in spelling of names, or inaccuracies in date of birth, but various record linkage programmes are available to identify “near matches” (Jones and Sujansky, 2004). These will be
ineffective, however, for people who have changed their name, e.g. because of marriage.
A further problem is that some countries do not have national death registrations, and these may be done on a regional or state basis instead, making it necessary to search multiple registers. Since 1979 a National Death Index for the United States has been compiled and
computerized and is available for vital status tracing (Wentworth et al, 1983).
Just because someone has been not been identified in death records, this does not mean that they are still alive and “at risk” since they may have emigrated or may not have been
identified in death registrations for some other reason. It is therefore desirable to confirm that they are alive using other record sources such as drivers license records, voter registrations, social security records, etc. In the United States, the Social Security
Administration (SSA) records have been frequently used in the past, and in Great Britain the Central Record Office of the Ministry of Pensions and National Insurance is the analogous tracing source (Checkoway et al, 2004).
Coding of the disease outcome It is not only necessary to determine if and when an event such as a death or hospital admission occurred. It is also necessary to verify, for example, the cause of death, or the cause of a hospital admission. Coding of causes of death should be performed by a nosologist trained in the rules specified by the International Classification of Diseases (ICD) volumes compiled by the World Health Organisation. In many countries this is done routinely for national death
registration records, and it is not
necessary (or desirable) to recode death registrations for a specific study.
However, the ICD codes have changed over time, and when using routine death registration records it is necessary to be aware of which ICD revision was in effect at the time of death.
Person-time
In a study of a specific population, e.g.
workers in a particular factory, participants may enter the study on the date that the study starts (1/1/70), or the date that they first
meet the eligibility criteria (i.e.
employment for one month),
whichever is the latest date. If they started working in the factory after the start of the study, then they would only start being followed on the date they started work (or a subsequent date when they met the eligibility criteria).
They stop contributing person-time when they die (or are diagnosed with the disease in an incidence study), emigrate, they are lost to follow-up, or the study finishes (31/12/99)
whichever is the earliest.
Example 9.4 Munk Nielsen et al (2003) studied long- term mortality after poliomyelitis by identifying a group of 5,977 patients diagnosed with poliomyelitis in Copenhagen between 1919 and 1954. This involved a review of more than 80,000 consecutive hospital records for
Blegdamshospitalet which served as the primary centre for diagnosing and treating patients with acute poliomyelitis in the area of greater Copenhagen.
Information extracted from the records
included name, sex, date and place of birth, date of admission and
discharge, and details of the acute severity of the case.
Since 1 April 1968, all Danish citizens have been given a unique identification number, which is recorded in the Danish Civil Registration System (CRS). The cohort was linked to the CRS to identity individual CRS numbers which were then used to identify deaths in the Danish Cause-of-Death Register. Patients not identified in the CRS were believed to have died or emigrated before 1 April 1968 and for these patients the Cause-of-Death Register was searched for their name and date of birth.
Patients were followed from the initiation of the Cause-of-Death Register in 1943 or the month after the hospital
discharge (whichever came later) until the date of death, emigration or 1 May 1997 (whichever came earlier).
There were 1295 deaths compared with an expected number of 1141 (SMR 1.14, 95% CI 1.07-1.20). Excess mortality was restricted to polio patients with a history of severe paralysis of the extremities (SMR = 1.69, 95% CI 1.32-2.15) or patients who had been treated for
respiratory failure during the epidemics (SMR = 2.71; 95% CI 2.18- 3.37).
Summary
Cohort studies provide the most
comprehensive approach for evaluating patterns of exposure and disease, since they involve studying the entire source population (assuming that there is a 100% response rate) over the entire risk period.
Thus, the cohort design ideally includes all of the relevant person-time
experience of the source population over the risk period. A cohort study may be based on a particular community (e.g. a geographical community), or on a more specific population defined by a
particular exposure (e.g. workers in a particular factory). In both instances, an internal comparison would ideally be made between those participants
exposed and those participants not exposed to a particular risk factor.
However, in some instances, all of the study participants may be exposed, or valid individual exposure information may not be available, and it may be necessary to make an external
comparison, e.g. with national mortality rates (in which case the national
population comprises the source
population for the study). It is important that any comparisons are made over the same risk period, and that follow-up is as complete as possible. The basic effect measures in a cohort study are the rate ratio and risk ratio. Methods of data analysis for these effect measures are described in chapter 12.
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