Changes in the surveillance system
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Sometimes surveillance data may show changes over time which are not real.
- If the proportion of patients with a disease who are seen in health facilities changes, the number of cases of reported disease will change without a real change in the number of cases in the population. Such changes can occur if health facilities are opened or closed leading to a rise or fall in the proportion of cases seen in facilities.
- Spurious changes can also occur if the case definition for the disease is changed. If the new case definition is broader so that it includes more cases of the disease, the number of reported cases may rise.
Between 1992 and 1993, the Centers for Disease Control and Prevention (CDC) changed the case definition for acquired immunodeficiency disease (AIDS) in the United States. The new definition added the result of a laboratory test to the list of conditions which defined a case as AIDS. If patients had a CD4 count less than 200 per microlitre, regardless of symptoms, their illness met the new definition of AIDS.
This graph shows the number of AIDS cases reported as heterosexually acquired in the United States by year between 1981-1993. If you didn't know about the change in case definition, you might be concerned that the AIDS epidemic was skyrocketing among heterosexuals between 1992 and 1993.
But the graph below distinguishes between those AIDS cases which fit only the old case definition (pre-1993) and those cases which met the new case definition only because they had a CD4 count less than 200 per microlitre (<200 CD4+ T-Lymphocytes/microlitre). This graph explains this apparent sharp rise in the number of cases as almost entirely due to the change in case definition.