Background:
Composite measures, like the Disease Activity Score for 28 joints (DAS28),
are key primary outcomes in rheumatoid arthritis (RA) trials. DAS28 combines
four different components in a continuous measure. When one or more of these
components are missing the overall composite score is also missing at
intermediate or trial endpoint assessments.
Objectives:
This study examined missing data patterns and mechanisms in a longitudinal RA
trial to evaluate how best to handle missingness when analysing composite
outcomes.
Design:
The Tumour-Necrosis-Factor Inhibitors against Combination Intensive Therapy
(TACIT) trial was an open label, pragmatic randomized multicentre two arm
non-inferiority study. Patients were followed up for 12 months, with monthly
measurement of the composite outcome and its components. Active RA patients
were randomized to conventional disease modifying drugs (cDMARDs) or Tumour
Necrosis Factor-α inhibitors (TNFis).
Methods:
The TACIT trial was used to explore the extent of missing data in the
composite outcome, DAS28. Patterns of missing data in components and the
composite outcome were examined graphically. Longitudinal multivariable
logistic regression analysis assessed missing data mechanisms during
follow-up.
Results:
Two hundred and five patients were randomized: at 12 months 59/205 (29%) had
unobserved composite outcome and 146/205 (71%) had an observed DAS28
outcome; however, 34/146 had one or more intermediate assessments missing.
We observed mixed missing data patterns, especially for the missing
composite outcome due to one component missing rather than patient not
attending thier visit. Age and gender predicted missingness components,
providing strong evidence the missing observations were unlikely to be
Missing Completely at Random (MCAR).
Conclusion:
Researchers should undertake detailed evaluations of missing data patterns
and mechanisms at the final and intermediate time points, whether or not the
outcome variable is a composite outcome. In addition, the impact on
treatment estimates in patients who only provide data at milestone
assessments need to be assessed.
Trial Registration ISRCTN Number:
37438295