Multiple Sclerosis Journal - October 2017 - 1511

JMA Wijnands, E Kingwell et al.

Figure 1. Post hoc analyses exploring sex differences across the five most common infection groups. Adjusted rate ratios
and 95% confidence intervals for infection-related hospitalizations and physician visits in people with multiple sclerosis
as compared with controls. The reference groups were the controls of the same sex. Adjusted factors: age (continuous),
year of index date (1996-2001, 2002-2007, and 2008-2013), SES (quintiles), immunotherapy use not specific to MS
(yes, no), and number of comorbidities (none, 1, and ⩾2).

combined, the magnitude of the effect was lower for
physician visits but remained similar for hospitalizations and prescriptions (Supplementary Appendix
e-2; Table e-3b).
Discussion
In this matched cohort study, we found that individuals with MS had higher rates of infection-related
health care utilization than the controls from the general population and that this was consistent across the
inpatient and outpatient care settings. People with MS
had an almost 240% increased risk of hospitalization,
over 40% more physician visits and nearly 60% more
antimicrobial drug prescriptions. Differences in health
care utilization were observed across all age groups,
with disproportionately higher utilization in men with
MS than women with MS.
While there are few studies with which to compare
our findings,3,4,6 three have explored infection-related
health care utilization in MS relative to individuals
without MS, all of which focused on hospitalizations.
Two studies examined the risk of a first hospitalization for an infection after MS diagnosis and reported
a 1.5-fold4 and >4-fold3 increase in people with MS,

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respectively. The former study used data from the US
Veteran Affairs health system and included predominantly male veterans with a mean age of 54 years. The
latter study from Sweden accessed population-based
data whose MS cohort had a similar age and sex distribution to ours. A Canadian study, using a similar
method to identify people with MS and matched controls, reported a 1.7-fold increased hospitalization risk
over a 25-year period; this included all infections captured in the ICD-10 chapter ("Certain infectious and
parasitic diseases") versus our current study which
also included additional organ-specific infections.6
We extend these findings by assessing multiple medical encounters over time rather than limiting to the
first hospitalization for an infection and by examining
medical encounters in the outpatient setting, including physician visits and prescriptions filled.
The considerable increases in infection-related encounters among people with MS included twofold increases
in hospitalizations for sepsis, skin infection, and intestinal infections. Sepsis represents one of the most
costly conditions treated in the hospital setting,
estimated at US$20 billion per year in the United
States alone,30 with substantial morbidity and mortality
risks for patients. The more than twofold increase in

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Table of Contents for the Digital Edition of Multiple Sclerosis Journal - October 2017