Dear Editor,

We read with interest the recent article by Araujo and colleagues evaluating electrocardiographic findings in paediatric patients with rheumatic diseases receiving hydroxychloroquine. ^{Reference Araujo, Araujo and Rufino1} The authors are to be commended for addressing a clinically important question in a population that has received limited dedicated investigation. However, we wish to raise two methodological concerns regarding the choice of heart rate correction formula and the absence of sex-stratified analysis, both of which may have implications for the interpretation of the reported findings.

The corrected QT (QTc) interval was calculated using the Bazett formula (QT/√RR). Although Bazett’s formula remains the most widely used method in routine clinical practice, its well-recognised limitation of overcorrection at higher heart rates is particularly relevant in the paediatric context. Children inherently exhibit greater heart rate variability and higher resting heart rates than adults, conditions under which Bazett’s formula is known to produce systematically elevated QTc estimates. A recent retrospective study comparing six QTc correction methods in a paediatric cardiology population demonstrated that the Fridericia formula (QT/∛RR) provided the best rate correction, as it showed the lowest correlation with both heart rate and the RR interval. ^{Reference Kwag, Kak, Li, Khass and McKechnie2,Reference Giudicessi, Noseworthy, Friedman and Ackerman3} Importantly, the present cohort included patients with systemic lupus erythematosus, a disease in which tachycardia may occur as a consequence of disease activity, pericarditis, or anaemia, yet no heart rate or RR interval data were reported.

A second concern relates to the absence of sex-stratified analysis. The study cohort was predominantly female (76.9%), consistent with the known epidemiology of systemic lupus erythematosus, yet the analysis did not account for established sex-related differences in QTc. It is well recognised that the QTc interval differs between sexes in a developmentally dependent manner. ^{Reference Gędos, Drygas and Kowalski4} As a consequence, accepted upper normal thresholds differ by sex, with QTc ≥ 450 ms considered prolonged in males and QTc ≥ 460 ms in females. ^{Reference Roden5} In the present study, a uniform threshold of QTc ≥ 450 ms was applied to all patients regardless of sex, which risks misclassifying borderline values in female patients and may have underestimated the proportion of subjects with clinically relevant prolongation. A large epidemiological study by Rautaharju and colleagues, encompassing more than 57,000 subjects aged 5–89 years, demonstrated that upper normal limits for QTc vary substantially by both age and sex, and recommended that age-and sex-specific criteria be applied when evaluating QT prolongation. ^{Reference Rautaharju, Mason and Akiyama6} Given the female predominance of the present cohort and the age range spanning prepubertal children through adolescence, a sex-stratified subgroup analysis would be an informative and methodologically appropriate addition.

We acknowledge the inherent difficulties of conducting prospective electrocardiographic surveillance in a rare paediatric rheumatological population, and we recognise the value of the data presented. Nevertheless, reanalysis using the Fridericia correction formula alongside sex-stratified QTc thresholds would meaningfully refine the precision of the safety conclusions.