IHP 330 Module 2 Measures of Disease Frequency Paper example

Few New, Many Living With It: Calculating the Incidence, Prevalence and Mortality of Multiple Sclerosis in a Composite County

[Student Name]

Southern New Hampshire University

IHP 330: Principles of Epidemiology

Module Two Assignment

[Instructor Name]

[Date]

The organization, setting and figures below are a composite written as a model document. No real employer, client, colleague or patient is described.

Few New, Many Living With It: Calculating the Incidence, Prevalence and Mortality of Multiple Sclerosis in a Composite County

The Planning Question and the Data

A composite regional health system serving a county of 142,000 adults runs one multiple sclerosis clinic with two neurologists and a small infusion suite for disease-modifying therapies. Waiting times for new appointments have reached four months, and leadership wants to know how many county adults are living with the disease, how many are newly diagnosed each year and what the pattern means for staffing. From the clinic's registry and the region's neurology records, the analyst found that 357 adult residents had a diagnosis of multiple sclerosis on January 1, 9 were newly diagnosed during the year, 6 people with the disease died, and 360 were living with it on December 31. The analyst assumed no patient relocated in either direction over the twelve months, which keeps the arithmetic simple.

Incidence

Incidence divides the new cases that arise over a stated time by the number of people who could have become cases, then scales the result to a base such as 100,000 (Centers for Disease Control and Prevention [CDC], 2012). People already living with the disease are not at risk of a first diagnosis, but they are so few relative to the population that the total adult population is an acceptable denominator.

Incidence = 9 new cases / 142,000 adults x 100,000 = 6.3 per 100,000 adults per year. Only about 6 of every 100,000 adults were diagnosed during the year.

Prevalence

Point prevalence is the proportion of a population with the condition at a specific time. On December 31, 360 adults were living with multiple sclerosis.

Prevalence = 360 / 142,000 x 100,000 = 253.5 per 100,000 adults. About one adult in every 400 in the county has the disease. Prevalence is 40 times incidence, and it is prevalence, not the handful of new diagnoses, that determines how many patients the clinic must follow every year.

Mortality Among People With the Disease

The death rate among people with multiple sclerosis uses patients as the denominator. Because the group changed during the year, the average number living with the disease approximates person-years at risk: (357 + 360) / 2 = 358.5.

Mortality rate among patients = 6 deaths / 358.5 person-years x 100 = 1.7 deaths per 100 patient-years. For the whole adult population, the corresponding rate is 6 / 142,000 x 100,000 = 4.2 per 100,000. The first figure describes prognosis for patients; the second describes the population burden. Neither shows that multiple sclerosis caused the deaths, since the registry counts all deaths among people with the diagnosis.

Rates by Age and Sex

Crude rates can hide large differences between groups. The table below shows specific incidence rates.

Table 1

Age- and Sex-Specific Incidence of Multiple Sclerosis

Note. Each rate equals new cases divided by the group's population, times 100,000. Small numbers make these rates unstable from year to year.

Reading the Specific Rates

New diagnoses cluster among younger and middle-aged adults and among women, whose incidence is more than three times that of men. The prevalence pattern follows: 270 of the 360 people living with the disease are women, a prevalence of 375 per 100,000 women compared with 129 per 100,000 men. With only nine new cases, however, a single year's specific rates can swing widely, so the analyst should pool several years before drawing firm conclusions about any group.

Why the Crude Rate Alone Would Mislead

Had the analyst reported only the crude incidence of 6.3 per 100,000, leadership might have concluded that multiple sclerosis is too rare to plan around. The specific rates and the prevalence tell a different story. A disease that strikes mainly young women and lasts for decades creates a large, steady population of patients who need regular neurology care, imaging every year or two, and in many cases infusions every few weeks or months. Comparing the county with others would also require care: a county with a younger, more female population would show a higher crude incidence even if its age- and sex-specific risks were identical. For comparisons of that kind, epidemiologists use rates adjusted to a standard population, which remove the effect of differing population structures before any conclusion is drawn about risk.

Connecting Incidence and Prevalence

In a population in steady state, prevalence approximately equals incidence multiplied by the average duration of the disease (Celentano & Szklo, 2019). Here, 253.5 / 6.3 gives an average duration of roughly 40 years. That figure is only approximate, because the county is not in a perfect steady state and some people diagnosed elsewhere will have moved in, but it captures the key fact: multiple sclerosis is usually diagnosed in early adulthood and lived with for decades. Each new diagnosis therefore adds decades of clinic visits, imaging and often infusion therapy.

Comparison and Planning Implications

A national population-based study estimated that about 745,000 adults in the United States were living with multiple sclerosis, with roughly three quarters of them women, and found that prevalence rose with latitude and differed by race and ethnicity (Hittle et al., 2023). The county's prevalence is in the same general range as national estimates, which suggests the registry is capturing most patients rather than revealing an unusual cluster.

For planning, the numbers support expanding follow-up capacity rather than diagnostic capacity. With 360 patients who typically need at least one to two neurology visits a year and many on infusion therapies, two neurologists are stretched thin. Adding an advanced practice provider for stable follow-up visits and extending infusion hours would address the burden that prevalence reveals.

Limits of the Data

The registry may miss people treated outside the region, and diagnosis dates may be recorded inconsistently. The assumption of no migration is unrealistic. Small annual counts make incidence unstable. A stronger analysis would use several years of data and link to state or claims records.

Conclusion

The calculations show an incidence of 6.3 per 100,000 adults, a prevalence of 253.5 per 100,000, a death rate of 1.7 per 100 patient-years among people with the disease, and higher incidence among women and younger adults. Because the disease lasts decades, prevalence is about 40 times incidence, and it is prevalence that should guide the clinic's staffing decisions.

References

Celentano, D. D., & Szklo, M. (2019). Gordis epidemiology (6th ed.). Elsevier.

Centers for Disease Control and Prevention. (2012). Principles of epidemiology in public health practice: An introduction to applied epidemiology and biostatistics (3rd ed.). U.S. Department of Health and Human Services.

Hittle, M., Culpepper, W. J., Langer-Gould, A., Marrie, R. A., Cutter, G. R., Kaye, W. E., Wagner, L., Topol, B., LaRocca, N. G., Nelson, L. M., & Wallin, M. T. (2023). Population-based estimates for the prevalence of multiple sclerosis in the United States by race, ethnicity, age, sex, and geographic region. JAMA Neurology, 80(7), 693-701. https://doi.org/10.1001/jamaneurol.2023.1135