Meningococcal disease is a serious life threatening infection that can be passed on to a patient’s close contacts. In many parts of the world, the role of health departments is to rapidly identify close contacts who may be at increased risk so that they can be counselled and, if necessary, offered clearance antibiotics to prevent ongoing transmission. In New York state, in 1991 this surveillance system relied on doctors notifying cases to county health departments. Because of the importance of preventing the spread of this nasty disease, New York State Health Department epidemiologists wanted to see how complete the surveillance system was for picking up every case of meningococcal disease.
To do this, they compared the names of patients notified to selected county health departments with the names of patients recorded in hospital records as having been admitted with the disease, and calculated: how many patients were captured by each system and, from these numbers, estimated the total number of patients with meningococcal disease likely to have been diagnosed in New York that year, using the capture-recapture method.
They estimated that 107 cases of meningococcal disease occurred in the selected counties in 1991, but found only 100 were notified with the disease.
According to the US Centers for Disease Control and Prevention, the sensitivity of a surveillance system refers to the proportion of cases of a disease detected by the surveillance system.
In this example, the notification system picked up 100 of the estimated 107 cases of meningococcal disease in New York state, giving a sensitivity of 93%.