Fig. 2: Linearity assessment.

Evaluation of the non-linear relationship between the biomarkers and A Long COVID or B PACS using a natural cubic spline model. Error bands were calculated using 95% confidence intervals. The two-sided P-value was calculated using ANOVA. Red colour indicates a P-value smaller than 0.05, indicating that the non-linear model is a better fit compared to the linear model. Blue colour indicates a P-value higher than 0.05, suggesting that the non-linear model and the linear model are similar. Note: Notice that the non-linear test was performed between the log-odds of the outcome model, not the probability of the outcome itself. However, the probability of the outcome is plotted as it is more intuitive and accessible for interpretation. Notice that the shape of the curve is similar when plotted on the log-odds scale.