Diabetes Increases Risk of Active TB

By Michael Smith
BOSTON, 16 july 2008-- Diabetes together with a tuberculosis infection increases the risk of active TB by a factor of three, researchers here said.
The finding comes from a systematic review of 13 observational studies looking at the link between diabetes and TB over the past four decades, according to Christie Jeon, M.B., and Megan Murray, M.D., D.P.H., of the Harvard School of Public Health.
In places such as India and China, where the background prevalence of TB is high, diabetes could account for as much as 10% of active cases, the researchers said in the online journal PLoS Medicine.
"We found consistent evidence for an increased risk of TB among people with diabetes," they said.
What's more, the rise in type II diabetes -- the current 180 million cases is predicted to rise to 366 million by 2030 -- "does not bode well for the future" of TB control, the researchers said.
Physicians have noted an association between diabetes and active TB since the early part of the last century, but it has not been clear whether diabetes caused the TB or vice versa.
To help clarify the issue, the researchers conducted a systematic review of studies published from 1965 to 2007 that gave an age-adjusted quantitative estimate of the association between diabetes and active TB.
All told, they found 13 such studies, with 1.7 million participants -- three prospective cohort studies, eight case-control studies, and two in which TB case accrual was prospective, but the underlying distribution of diabetes was determined later.
All three of the cohort studies -- conducted in India, South Korea, and Taiwan -- found a significant association between diabetes and active TB, with a summary relative risk of 3.11 and a 95% confidence interval from 2.27 to 4.26.
All eight case-control studies also showed an effect, with relative risks ranging from 1.16 to 7.83, but two of the confidence intervals crossed unity.
One of the remaining two studies showed a relative risk of 6.0, with a 95% confidence interval from 5.0 to 7.2, but the other showed no effect, the researchers found.
Heterogeneity -- or between-study variance -- accounted for 39% of the total variance among the cohort studies, 68% among case-control studies, and 99% in the remaining two studies.
Despite that heterogeneity, the researchers said, the studies show "there is an increased risk of active TB among people with diabetes regardless of background incidence, study region, or underlying medical conditions in the cohort."
They cautioned that the summary estimate of a 3.11 increase in relative risk is derived from cohort studies conducted in Asia, and may be different in other populations, depending on such variables as background TB prevalence and age.
Nevertheless, "it is clear that (diabetes) constitutes a substantial contributor to the current and future global burdens of TB," they said.
For example, assuming a relative risk of 3.0 and a 6% prevalence of diabetes in Mexico, the researchers calculated that diabetes accounts for 67% of active TB cases among those with diabetes, and 11% of cases among the entire Mexican population.
The researchers noted that the analysis has several limitations, including difficulty accounting for possible confounding factors in the underlying studies and selection biases.
The researchers did not report any external sources of support for the study. They said they had no conflicts.
Primary source: PLoS MedicineSource reference:Jeon CY, Murray MB "Diabetes mellitus increases the risk of active tuberculosis: A systematic review of 13 observational studies" PLoS Med 2008; 5(7): e152. DOI: 10.1371/journal.pmed.0050152.