Which of the following refers to the incidence rate of dementia in a catchment area?
C. The incidence of a disease is defined as the number of ‘new’ cases diagnosed in a specified time interval for a specified size of population at risk. The midinterval population usually determines this population size. For example while calculating the incidence of a disease in 1 year, the comparison is made against the midyear population.
Incidence is a rate ratio, that is it is measured against time. It is not a mere number and it is usually expressed per 100 000 persons in a population, per year. The essential criterion is that the measure should indicate all new occurrences of a disease within the period of observation in an area, irrespective of whether the newly diagnosed patients are cured or dead within the period of observation itself. For an accurate measurement of incidence, two cross-sectional surveys must be carried out in the same population; one must be at the beginning of a defined period and the other at the end of the same period.
In a National Comorbidity Survey carried out in the US, the proportion of the sampled individuals who ever manifested criteria of panic disorder in their lifetime was determined.
This can be best described as which of the following?
C. Lifetime prevalence is the proportion of individuals in the population who have ever manifested a disorder, who are alive on a given day. This is ascertained by surveying a population cross-sectionally and finding out if they ever satisfied the criteria for a disorder in the past or at the present time. As one can observe, although this method is commonly used in epidemiological surveys, it is prone to recall bias. Lifetime morbid risk refers to risk of contracting a disease for each individual in a birth cohort if they live long enough to reach the average life expectancy of the population. This must be clearly differentiated from prevalence estimates. Prevalence is largely a population measure, while lifetime morbid risk is more close to an individual’s chances of being diagnosed with an illness.
Using case records, the number of newly diagnosed cases of psychosis in south-east London was determined for a period of 33 years, between 1965 and 1997. To calculate incidence rate, which of the following is the most suitable denominator?
C. Incidence is a ratio between the number of newly diagnosed cases within a specified time period in a population and the total number of people living in the area (total population). To be accurate, such comparisons must exclude those who are not at risk, though this is generally not done for non-infectious, non-epidemic diseases such as psychiatric illnesses. It is essential that the denominator and numerator are not mutually exclusive, that is the diseased group must be a part of the studied population. Hence, when measuring the incidence of psychosis, the population above the age of 16 is the relevant denominator. The year 1981 is the midinterval period between 1965 and 1997.
Which of the following equations gives the relationship between prevalence and incidence?
A. Prevalence is defined as the number of ‘existing’ cases in a specified population for a period of observation (either cross-sectional observation, called point prevalence, or longitudinal observation for a specified time, called period prevalence). The existing cases include all new cases and all cases diagnosed before the observation but still suffering from the disease, but existing cases excludes those who have been previously diagnosed but are now cured or dead. For illnesses that are significantly chronic (e.g. schizophrenia), prevalence will be higher compared to those illnesses that are acute and short lived (e.g. influenza), even if the incidence rates are comparable. Hence the simple expression:
prevalence = incidence × duration of illness
explains the relationship between incidence and prevalence.
A major cause of mortality in schizophrenia is cardiovascular problems. If a new class of antipsychotics with favourable metabolic profile that reduces cardiac risk is introduced, which of the following could happen?
C. Certain factors can influence incidence and prevalence differently. For example if a new vaccine is developed to prevent an illness, both incidence and prevalence may come down. If a cure is developed for schizophrenia, incidence may not be affected but prevalence could drop. Similarly, if interventions are introduced to reduce mortality in chronic schizophrenia, then prevalence may paradoxically increase due to longevity of patients. This may not affect incidence rates directly.
Higginson IJ and Constantinin M. Epidemiology of symptoms in advanced illness. In: Max MB and Lynn J, eds. Symptom Research: Methods and Opportunities. http://symptomresearch.nih.gov/ tablecontents.htm (accessed 19.8.08).
© 2010-2030 Your Doctor - Dr.Khalil Al-Yousifi - Kuwait - Contact Us