Statistical analysis
All analyses were performed with the IBM Statistical Package for the
Social Science version 25.0 (SPSS Inc., Chicago, IL, USA) for MacOS.
Descriptive data were given as percentages and as median (IQR 25-75).
The Spearman’s correlation test was used to confirm the correlations
between CG versus (vs.) serum immunoglobulins and gamma globulin vs.
serum immunoglobulins. The power of correlation was defined as very weak
if r <0.2, weak if r=0.2–0.4, moderate if r=0.4–0.6, strong
if r=0.6–0.8 and very strong if r>0.8.
Receiver operating characteristic (ROC) curve analyses were performed to
verify if CG and gamma globulin were independent discriminative values
to detect subjects with IgG serum levels lower than 600, 500, 400 and
300 mg/dL. The accuracy of the obtained discriminant value was
interpreted based on the area under curve (AUC). The Youden’s index was
calculated to confirm the discriminant score, defined as the highest
value observed for the following operation: sensitivity + specificity –
110. Since CG was designed as a screening test, the
cut-off value which has a sensitivity of >80% was accepted
as the most appropriate among the values with the highest Youden’s
index10,11.
The chi-square test was applied to compare the frequency of different
clinical characteristics between the CG value of <2.0 and
≥2.0. Significant variables were further evaluated using the logistic
regression analysis. The results were assessed at a significant level of
p<0.05 and a 95% confidence interval (CI).