Question

1. When would it be important to distinguish between the incidence of a specific disease or condition in your patient population (any patient population) and the prevalence of the disease/condition.

2. What is the sensitivity and specificity of a diagnostic test used to make diagnoses for your patient population (used in #1) ? Discuss the implications of the values you present.

Answer 1

1) Incidence of a disease (in epidemiology) is a measure of probability of occurrence of a medical condition or a disease in a population within a specified period or time. It is expressed simply as the number of new cases during some time period.

Incidence = New cases / Population at risk

whereas,

Prevalence of a disease (in epidemiology) is the proportion of a particular found to be affected by a medical condition or a disease at a specific time. It is derived by comparing the number of people found to have the condition with the total number of people.

Prevalence = Existing cases   / Population at risk.

# Prevalance and incidence are both confused frequently. So prevalence and incidence are similar, but prevalence includes new and pre-existing cases whereas incidence includes new cases only.
* When we need to describe how often a disease or another health event occurs in a population. When  the focus of the study is the occurrence and determinants of disease. Measuring the frequency of a disease or other health outcome in a population and identifying how the disease frequency may differ over time or among subgroups are important steps in discovering potential causes of a disease and determining effective methods for prevention and care.
When we need to know about the epidemicity of the disease and about the rate of occurance, risk and widespread of the disease, we need to distinguish Incidence and Prevalence.

** Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence  indicates how widespread the disease is.

2) In medical diagnosis,

Sensitivity of a diagnostic test is the ability of a test to correctly identify those with the disease i.e. True Postive Rate. It is the probability that a patient tests positive given that they have the disease.

Whereas,

Specificity of a diagnostic test is the ability of the test to correctly identify those without the disease i.e. True Negative Rate. It is the probability that a patient tests negative given that they do not have the disease.

# Higher prevalence - lower specificity.

Higher prevalence - higher sensitivity.