An epidemiological investigation was begun on July 1, 2000 among a population of
ID: 3367556 • Letter: A
Question
An epidemiological investigation was begun on July 1, 2000 among a population of 1,000 individuals. Three individuals were found to have leukemia on July 1st. During the ten-year follow-up period, five new cases of leukemia were diagnosed. Among the eight leukemia cases, four deaths occurred during the ten-year follow-up period. Two additional individuals, neither of whom had leukemia, were lost to follow-up at some point during follow-up. None of the remaining 990 study participants became ill, died, or were lost to follow-up during the follow-up period.
The diagram below indicates the experience of the 10 study participants, using the following symbols:
case
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
1
---
--*--
-----
-----
-----
-----
-----
-----
-----
-----
--X
alive
2
---
------
-----
--*--
-----
-----
-----
-----
-----
-----
--X
alive
3
*<--
------
------
-----
-----
-----
-----
-----
--X
dead
4
---
------
-----
-----
--X
lost
5
---
------
-----
-----
-----
-----
--*--
--X
dead
6
---
------
-----
-----
-----
-----
-----
-----
-----
--*--
--X
alive
7
---
--*--
-----
-----
--X
dead
8
*<--
------
------
-----
-----
-----
-----
-----
-----
-----
-----
--X
alive
9
---
------
-----
-----
-----
-----
-----
--X
lost
10
*<--
------
------
--X
dead
What was the prevalence of leukemia on January 1, 2005?
What was the cumulative incidence of leukemia during the 10-year follow-up period?
What was the incidence rate of leukemia during the 10-year follow-up period?
What was the case fatality rate during the 10-year follow-up period?
* :case recognition (diagnosis of leukemia) *< :onset of leukemia was prior to start of study period ---- :observation period during follow-up X :status at termination of observation (i.e., alive, dead, lost)Explanation / Answer
Based on the experience of 10 study participants,
1. By prevalence of leukemia on January 1,2005, one means point prevalence.It is given by,
Prevalence = No. of cases (both old and new) / Population size
= 6 / 10 = 0.6
(The count of cases obtained by adding the * and *< before 2005)
2.Cumulative incidence = No. of new cases during the 10 year period / No. initially at risk during the period
= 5 / 10 = 0.5
3. Incidence rate = No. of new cases / Total 'at risk' observation time (or disease free years)
= 5 / 31 = 0.1613
(The denominator here is obtained by count the no .of periods (-----) before *, excluding the three (*<) old cases)
4. Fatality rate = No. of deaths / No. of cases
= 4 / 8 = 0.5
(Here numerator is the no.of 'X' s for * and *< cases before the 10 year period and the denominator is the no. of * and *< cases during the 10 year period.)