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Evaluating bias with loss to follow-up in a community-based cohort: empirical investigation from the CARRS Study
  1. Dimple Kondal1,
  2. Ashish Awasthi1,2,
  3. Shivani Anil Patel3,
  4. Howard H Chang3,
  5. Mohammed K Ali3,4,
  6. Mohan Deepa5,
  7. Sailesh Mohan1,2,
  8. Viswanathan Mohan6,
  9. K M Venkat Narayan3,
  10. Nikhil Tandon2,7,
  11. Dorairaj Prabhakaran1,2
  1. 1 Centre for Chronic Disease Control, New Delhi, India
  2. 2 Centre for Chronic Conditions and Injuries, Public Health Foundation of India, New Delhi, India
  3. 3 Emory Global Diabetes Research Center,Woodruff Health Sciences Center, Emory University, Atlanta, Georgia, USA
  4. 4 Department of Family and Preventive Medicine, School of Medicine, Emory University, Atlanta, Georgia, USA
  5. 5 Epidemiology, Madras Diabetes Research Foundation, Chennai, Tamil Nadu, India
  6. 6 Diabetology, Madras Diabetes Research Foundation and Dr Mohan’s Diabetes Specialities Centre, Chennai, Tamil Nadu, India
  7. 7 All India Institute of Medical Sciences, New Delhi, India
  1. Correspondence to Dr Dimple Kondal, Centre for Chronic Disease Control, New Delhi, Delhi 110016, India; dimple{at}


Background Retention of participants is a challenge in community-based longitudinal cohort studies. We aim to evaluate the factors associated with loss to follow-up and estimate attrition bias.

Methods Data are from an ongoing cohort study, Center for cArdiometabolic Risk Reduction in South Asia (CARRS) in India (Delhi and Chennai). Multinomial logistic regression analysis was used to identify sociodemographic factors associated with partial (at least one follow-up) or no follow-up (loss to follow-up). We also examined the impact of participant attrition on the magnitude of observed associations using relative ORs (RORs) of hypertension and diabetes (prevalent cases) with baseline sociodemographic factors.

Results There were 12 270 CARRS cohort members enrolled in Chennai and Delhi at baseline in 2010, and subsequently six follow-ups were conducted between 2011 and 2022. The median follow-up time was 9.5 years (IQR: 9.3–9.8) and 1048 deaths occurred. Approximately 3.1% of participants had no follow-up after the baseline visit. Younger (relative risk ratio (RRR): 1.14; 1.04 to 1.24), unmarried participants (RRR: 1.75; 1.45 to 2.11) and those with low household assets (RRR: 1.63; 1.44 to 1.85) had higher odds of being lost to follow-up. The RORs of sociodemographic factors with diabetes and hypertension did not statistically differ between baseline and sixth follow-up, suggesting minimal potential for bias in inference at follow-up.

Conclusion In this representative cohort of urban Indians, we found low attrition and minimal bias due to the loss to follow-up. Our cohort’s inconsistent participation bias shows our retention strategies like open communication, providing health profiles, etc have potential benefits.


Data availability statement

Data are available upon reasonable request.

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Data availability statement

Data are available upon reasonable request.

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  • DK and AA are joint first authors.

  • Twitter @INDIACCDC, @AwasthiAshish15, @shivania0115, @mkali_twindad, @thePHFI, @drmohanv, @kmvnarayan14, @DrDPrabhakaran

  • DK and AA contributed equally.

  • Contributors Conceptualisation—DP, NT, MKA, KMVN, DK, AA, HHC and SAP. Data curation—DK and AA. Formal analysis—DK and AA. Writing (original draft)—DK and AA. Writing (review and editing)—SAP, HHC, DP, NT, MKA, DK, AA, MD, SM, VM and KMVN. All authors have read and agreed to the published version of the manuscript. DP is the guarantor for this paper.

  • Funding The CARRS Study was funded in part by the National Heart, Lung, and Blood Institute (NHLBI), National Institutes of Health (NIH), Department of Health and Human Services (contract no. HHSN268200900026C) and the United Health Group, Minneapolis, Minnesota, USA. KMVN, MKA and SAP were funded in part by the National Institute of Diabetes and Digestive and Kidney Diseases of the NIH (award number P30DK111024). KMVN was funded in part for 'Worksite Lifestyle Program for Reducing Diabetes and Cardiovascular Risk in India' Project funded by NHLBI, NIH Department of Health and Human Services (award number R01HL125442). SM,SAP, KMVN, MKA, NT and DP were supported in part by the NIH (award number 5U01HL138635) under the Hypertension Outcomes for T4 Research within Lower Middle-Income Countries (Hy-TREC) Programme. DK has been supported by Fogarty International Center for PH leader Course, NIH (grant number D43TW009135). Research reported in this publication was supported by the NHLBI of the NIH (award number P01HL154996-PCARRS).

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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