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 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)

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?

Answer 1

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.)