Bias due to incomplete follow up in a cohort study
P M Pennefathera, W Tinb, M P Clarkea, J Duttonc, S Fritzb, E N Heyb
a Department
of Ophthalmology, Royal Victoria Infirmary, Newcastle upon Tyne, b Regional Maternity Survey Office, Newcastle upon
Tyne, c Department of Clinical Audit, Southport and
Formby District General Hospital, Southport
Correspondence to: P M Pennefather, Royal Liverpool Children's Hospital, Eaton Road, Liverpool L12 2AP.
Accepted for publication 8 February 1999
AIM
To investigate the
bias introduced by incomplete follow up in a cohort study of ocular
outcome after premature birth.
METHODSA
geographically defined cohort of children born before 32 weeks'
gestation was prospectively recruited at birth to study the ocular
outcome at 2 years. On the basis of attendance at 2 years, the
children's families were allocated to one of three groups: group 1 attended for follow up, group 2 were difficult to trace, and group 3 were very reluctant for assessment. All children were examined by a
single ophthalmologist, masked to these groupings.
RESULTS558 children
(98.8% of study group) were examined, of whom 505 were in group 1, 20 in group 2, and 33 in group 3. The groups which were more difficult to
study (groups 2 and 3) showed a significantly higher prevalence of
ocular abnormalities, including strabismus (p=0.02) and cicatricial
retinopathy of prematurity (p=0.002) compared with those attending for
follow up. Further, not all of these cases could have been identified
by review of the children's previous records. Ocular abnormalities
would be underestimated by 16% (11.3% in group 1 compared with 13.4%
in the total cohort, p=0.77).
CONCLUSIONSThis study
suggests that the prevalence of abnormalities would be underestimated
by incomplete follow up, as those subjects who were most difficult to
obtain for study had a significantly higher prevalence of abnormalities.
© 1999 by British Journal of Ophthalmology
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