Longitudinal studies in sickle cell disease : using a prospective cohort to examine definitions and clinical course in sickle-cell disease epidemiology : study participants, haematology, clinical events and health status

Abstract

We present our work as four related ‘books’ covering study participants, haematology, clinical events, and health status.  The sickle-cell unit clinic load has increased from 3,668 presentations in 1973-75 to 35,682 in 1997-99.  The number of patients and the number of presentations per patient per year have both increased.  Non-enrolment among people with unscreened SS disease is considerable;  up to 90% will not enrol by five years of age and so will not benefit from important early-life clinical interventions, and up to half may not enrol for specialist clinical care during childhood.  Among SS disease patients that do enrol, over half do so in adolescence and adulthood when clinical management is less focused on preventive care.  In the JSSCD, 2.7% of participants default from the clinic for one complete calendar year, and 21.9% default from a single pre-arranged appointment.  Defaulting predicts subsequent mortality, which suggests an avenue for intervention.  Participants that default or dropout have different levels of haematology and different clinical expressions:  defaulters that survive have a mild haematological profile, while defaulters that subsequently die exhibit increased reticulocytosis.  This ‘non-ignorable’ non-response has far reaching consequences for how we analyse observational studies in sickle-cell disease.  Health status literature remains contradictory, and from 77 identified articles just 3 met minimal criteria for acceptable design.  From 16 articles offering systems for classifying health status a single article maintained some ability to minimise false-positive identification of adversely affected patients, but its general predictive ability degraded from 79% to 6%.  Our clinical case studies present a range of findings and conclusions, which we present in the body of our report. Our major case study investigated lifetime survival in SS disease, and we report an encouraging median survival of 53 years (95% confidence interval 49.0 to 58.7) for men and 58.5 years (53.5 to 70.9) for women.

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