Laboratory parameters are critical prognostic parameters in prognostic studies on SCD. We therefore conducted a review of the methods used in studies in this field to take into account and to analyse laboratory parameters. Our review found a large heterogeneity of the methods used. This led us to assess four different methodological strategies for analysis of laboratory parameters. We found that different methodologies can lead to different conclusions on the prognostic value of the laboratory parameters on CV.
There was an heterogeneity among the studies regarding the age range at the study inclusion. Given the variations in laboratory parameters depending on age [15] [16], as well as the absence of standards according to age and sex in the SCD population, the broad age ranges at study inclusion could weaken the study conclusions by inducing an error in the association estimation between the laboratory parameters and a SCD clinical event. For example, in assessing the association between fetal hemoglobin and CV in children between 18 months and 5 years of age, which is the period of onset of this complication, it is important to bear in mind that blood fetal hemoglobin levels decrease by approximately 25–15% over this age range. Moreover, blood hemoglobin levels vary depending on age with very high levels in newborns, followed by a gradual decrease, with the lowest levels observed between 1 and 6 months of age, followed by a gradual increase and then stabilisation at puberty [15].
Surprisingly, most cohort studies used a single value per patient in the statistical analysis, measured at various ages depending on the studies. When a full set of measures was available, one strategy applied in published studies to determine risk versus protective factor was to take into account the mean of the measures. However, with this strategy, changes over time in the laboratory parameter are not taken into account. For example, in the study conducted by Curtis et al [17], which enrolled 359 patients with sickle cell anaemia, lower baseline levels of fetal hemoglobin were associated with increased mortality, however a significant risk of increased mortality, based on longitudinal changes in percentage of fetal hemoglobin, was not detected.
The course of a laboratory parameter over time may further influence the different conclusions concerning the correlation between the laboratory parameter and the SCD event depending on the method of analysis. This could explain in part the variability of the results in the published studies. For example, fetal hemoglobin was shown to be protective against CV in one study [9] while other studies failed to demonstrate this [6] [7]. The difference in the results might be due to differences in the study populations or to differences in the definitions used, however it could also be due to differences in the methods used to assess the prognostic value of fetal hemoglobin.
To our knowledge, this is the first work that explores how laboratory parameters are used to predict outcome in SCD studies in children. Although this field is sparsely explored, it raises fundamental questions about methodological approaches relevant for clinician researchers. Even if they could request statistician support, they should be made aware of the importance in taking into account longitudinal data in prognostic cohort studies.
One of the strengths of this study is that suggests the use of an under-known method for statistical analysis that takes into account longitudinal data in the research of association with a clinical event, as has been done for other diseases, notably for Human Immunodeficiency Virus [11] [13] [18] [19] [20]. More recently, this method has been adopted in others areas of clinical research, including cancer [21], cardiovascular diseases [22], and kidney transplantation studies [23] [24] .In the field of SCD, this rigorous method was used for the first time only recently [9]; it took into account variations over time for repeated measures throughout the study.
Eligible studies were identified through one important database and only major journals were screened. We consider that we have found sufficient evidence to draw robust conclusions on the potential impact of the heterogeneity of measurement methods and strategies on the analysis of laboratory parameter.
Standardization of practices and methods in prognostic studies in children with SCD would help to avoid so-called vibration of effects, whereby results can differ, i.e. vibrate over a wide possible range, depending on how the analysis was conducted [25]. Standardization and homogenization of methods could lead to more transparent reporting in these studies and enhance the reliability of published data [26]. Large consortia and collaborations would allow investigators to use a common language for clinical definitions, laboratory measurements and statistical analysis.