Soodabeh Navadeh
1,2 
, Ali Mirzazadeh
1,3, Willi McFarland
3,4, Phillip Coffin
4,5, Mohammad Chehrazi
6, Kazem Mohammad
6, Maryam Nazemipour
7, Mohammad Ali Mansournia
6* , Lawrence C McCandless
8, Kimberly Page
91 Global Health Sciences, University of California, San Francisco, CA, USA
2 HIV/STI Surveillance Research Center, and WHO Collaborating Center for HIV Surveillance, Institute for Futures Studies in Health, Kerman University of Medical Sciences, Kerman, Iran
3 Department of Epidemiology and Biostatistics, University of California, San Francisco, CA, USA
4 San Francisco Department of Public Health, San Francisco, CA, USA
5 Division of HIV, ID, and Global Health, School of Medicine, University of California, San Francisco, CA, USA
6 Department of Biostatistics and Epidemiology, School of Medicine, Babol University of Medical Sciences, Babol, Iran
7 Psychosocial Health Research Institute, Iran University of Medical Sciences, Tehran, Iran
8 Faculty of Health Sciences, Simon Fraser University, Burnaby, BC, Canada
9 Department of Internal Medicine, Division of Epidemiology, Biostatistics and Preventive Medicine, University of New Mexico Health Sciences Center
*Corresponding Author: *Corresponding Author: Mohammad Ali Mansournia; Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, PO Box: 14155-6446, Tehran, Iran. Tel: +98-21-88989123; Fax: +98-21-88989127; Email: , Email:
mansournia_ma@yahoo.com
Abstract
Background: To apply a novel method to adjust for HIV knowledge as an unmeasured confounder for the effect of unsafe injection on future HIV testing.
Methods: The data were collected from 601 HIV-negative persons who inject drugs (PWID) from a cohort in San Francisco. The panel-data generalized estimating equations (GEE) technique was used to estimate the adjusted risk ratio (RR) for the effect of unsafe injection on not being tested (NBT) for HIV. Expert opinion quantified the bias parameters to adjust for insufficient knowledge about HIV transmission as an unmeasured confounder using Bayesian bias analysis.
Results: Expert opinion estimated that 2.5%–40.0% of PWID with unsafe injection had insufficient HIV knowledge; whereas 1.0%–20.0% who practiced safe injection had insufficient knowledge. Experts also estimated the RR for the association between insufficient knowledge and NBT for HIV as 1.1-5.0. The RR estimate for the association between unsafe injection and NBT for HIV, adjusted for measured confounders, was 0.96 (95% confidence interval: 0.89,1.03). However, the RR estimate decreased to 0.82 (95% credible interval: 0.64, 0.99) after adjusting for insufficient knowledge as an unmeasured confounder.
Conclusion: Our Bayesian approach that uses expert opinion to adjust for unmeasured confounders revealed that PWID who practice unsafe injection are more likely to be tested for HIV – an association that was not seen by conventional analysis.