4 Epidemiology Module - HIV in Katumba - Measuring Diseases

Alison Grant 28/8/18

Describing disease patterns is the first thing you need to do.

Use:

Prevalence - no of diseases in a population at a given point in time (sometimes called a point prevalence)

Incidence - no of new cases of disease in population over specified period of time

Prevalence will depend on incidence and duration (how long each case lasts for)

As HIV has a lifelong duration, the prevalence can be very high, despite the low incidence.

If prevalence is constant, and you know duration, you can calculate incidence P = I x D

4.1 Incidence

Incidence can be a cumulative incidence (incidence risk), or an incidence rate

All describe new cases over a specified time period. The difference is how you describe the denominator

Strictly the population should only be the population at risk.

For example, in calculating the incidence of a new disease, you shouldnt count the people already with it, because they arent at risk, they cant get it again.

New cases in a defined period of time, in a population at risk of disease at the start of a period of observation.

4.1.1 Cumulative incidence (ncidence Risk)

Numerator: number of people with a new diagnosis of disease during this time period denominator: total number of people in population at risk of disease at start of period of observation

New cases / people free of disease. expressed as percentage or cases per thousand

The problem with cumulative incidence is when people become lost to follow up. In cohort studies loss to follow up is often a problem.

4.2 Incidence Rate: Incidence/incidence density (person time at risk)

Incidence will depend on how many people you follow and how long you follow them up for.

If that population size changes you need to take that into account.

Now incidence rate = “person time at risk” - total time that every person at risk of disease is present during observation period. So time present until they leave the program, we get to the end of the period, or they acquire disease.

So it’s new cases, over this incidence rate.

This measure is way more useful for accounting for loss of follow up.

However:

The calculation of incidence rate relies on knowing when a patient becomes positive for a condition. IF we were trying to be fancy we should try and take that into account. By convention, if you are regularly measuring diseases regularly, and there’s a new case, you assume that it occured half way between this positive measure, and the last negative measure.