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Table 2 Risk of bias in included studies; each criterion was graded as high, low, or unclear risk

From: Association between low vitamin D levels and the diagnosis of asthma in children: a systematic review of cohort studies

Study name Developing and applying appropriate eligibility criteria Measurement of exposure Measurement of outcome Controlling for confounding Completeness of data
• Hollams [13] • Uncertain risk • High risk • Low risk • High risk • High Risk
  • Although the risk of bias is low for the original cohort, no further details were provided about selection criteria or process for participants in this current study • “Vit D levels was measured in thawed serum cryobanked at age 6 years” (number of years since blood draw not mentioned) • Low for lung function and BHR • Did not match or adjust for maternal atopy, maternal asthma, maternal age, education or household smoking • Outcome data were missing for 30% of the enrolled cohort
• The used enzyme immunoassay kit “method appeared to overestimate the vitamin D levels at age 6 years”
• Measuring Vitamin D levels at one point only may not be a reliable measure of integrated 25(OH) D levels over time
Van Oeffelen [12] • High risk • High risk • Low risk • Low risk • Uncertain Risk
• Out of the larger cohort, a small “selected” sample included in this study, with no further details about selection provided • Serum samples were defrosted to measure concentrations of Vitamin D (number of years since blood draw not mentioned) for asthma (ISAAC score) Low risk for BHR • Confounders were added to all models (gender, maternal atopy, paternal atopy, smoking by anyone in the house, and serum magnesium) • Outcome data were missing for 12% of the enrolled cohort
• Measured using a competitive enzyme immunoassay in microtiter plates
• Measuring Vitamin D levels at one point only may not be a reliable measure of integrated 25(OH) D levels over time • Also considered playing outside and overweight as potential confounders
Tolppanen [22] • Uncertain risk • High risk • Uncertain risk for asthma, using spirometry & bronchidilatory responsiveness with non-validated questionnaires to diagnose asthma • Low risk • High risk
• Except for the loss of follow up, the cohort was from a single community and followed specific eligibility criteria • The exposures are standardized for age and sex and 25-hydroxyvitamin D3 is adjusted for season and ethnicity • Model 1 unadjusted • Of 5765 participants in the assessment of the wheezing and asthma outcome 3323 where included (42% missing data)
• Measured using high pressure liquid chromatography tandem mass spectrometer in the multiple reaction mode (number of years since blood draw not mentioned) • High risk for wheezing • Models 2 and 3 adjusted for respectively 8 and 9 potential confounders
• Recall bias • And of 4488 participants in the spirometric assessment 2259 where included (50% missing data).
• Measuring Vitamin D levels at one point only may not be a reliable measure of integrated 25(OH) D levels over time • Proportion of incident wheezing, and asthma was higher among children excluded owing to missing data