Standardization of Aspergillus IgG diagnostic cutoff in Nigerians

Abstract

Background and objectives: Commercial Aspergillus IgG antibody assays have become pivotal in the current diagnosis of chronic pulmonary aspergillosis (CPA). However, diagnostic cutoffs have been found to vary from manufactures' recommendations in different settings. This study aimed to establish the Aspergillus IgG reference range among Nigerians and determine a diagnostic cutoff for CPA.

Methods: Sera from 519 prospectively recruited healthy blood donors and 39 previously confirmed cases of CPA were analysed for Aspergillus IgG levels using the Bordier test kit (Bordier Affinity Products SA, Crissier, Switzerland). Accuracy versus cutoff profile and receiver operating characteristics (ROC) curve were analysed for both CPA cases and controls using the R-Studio (2020), (Window desktop, version 4.0.2 software with R packages "nnet" and "ROCR").

Results: Among healthy blood donors, 141 (27.2%) were aged 16-25 years with median (interquartile range, IQR) of 22 (20-24) years; 304 (58.6%) were aged 26-40 years with median (IQR) of 32 (29-36) years; while 74 (14.2%) were aged 41-60 years with median (IQR) of 46 (44-49.75). Median IgG level in respective age groups were 0.069 (0.009-0.181), 0.044 (0.014-0.202) and 0.056 (0.01-0.265) with no significant difference found in the three age categories (p = 0.69). The overall diagnostic cutoff for the diagnosis of CPA was 0.821 with an accuracy of 97.1% and area under the curve (AUC) = 0.986.

Conclusion: The optimal diagnostic cutoff for diagnosing CPA in Nigerians using the Bordier kit was 0.821 which is lower than the manufacturer's recommended cutoff of 1.0. The determination of this cutoff among Nigerians will significantly enhance accurate identification of CPA and assessment of its true burden in Nigeria.

Keywords: Aspergillus IgG; Nigeria; blood donors; chronic pulmonary aspergillosis; cutoff.

Figure 1.

Flow diagram of healthy donor participants in the study (using STARD format).

Figure 2.

Stratified age, and gender IgG distributions: (a) is stratified age distribution, age group 16–25 (141 (27.2%), min = 17, max = 25, median (interquartile range) = 22 (20–24)); age group 26–40 (304 (58.6%), min = 26, max = 40, 32 (29–36)); age group 41–60 (74 (14.2%), min = 41, max = 57, 46 (44–49.75)). (b) is IgG level distribution by age stratification, age group 16–25 (min = 0.001, max = 1.324, 0.069 (0.009–0.181)), age group 26–40 = (min = 0, max = 3.24, 0.044 (0.014–0.202)) and age group 41–60 = (min = 0, max = 2.29, 0.056 (0.01–0.265)), there was no difference in IgG level across the age group, p = 0.69. (c) is age distribution by gender, female (F) = (107 (20.6%), min = 19, max = 55, 34 (26–38)), male (M) = (412 (20.6%), min = 17, max = 57, 30 (24.75–37)), difference between female and male age is significant, p = 0.035. (d) is IgG versus gender distributions, female (F) = (min = 0, max = 3.24, 0.139 (0.013–0.299)), male (M) = (min = 0, max = 2.23, 0.044 (0.012–0.181)). There was a significant difference between female and male IgG level, p = 0.0066.

Figure 3.

Frequency distribution of participants occupation. The median (IQR) of Aspergillus-specific IgG of artisans, traders, civil servant (CS), clergy, farmers, private sector (PS), professionals (prof), students, unemployed (unemp), and Others were 0.069 (0.016–0.191), 0.012 (0.011–0.196), 0.047 (0.011–0.194), 0.023 (0.022–0.026), 0.152 (0.081–1.02), 0.05 (0.012–0.199), 0.043 (0.01–0.149), 0.083 (0.013–0.232), 0.051 (0.012–0.189), and 0.019 (0.0–0.089), respectively.

Figure 4.

IgG level stratified by geographical regions. Median (IQR) values in the regions are north central (NC) (n = 101 (19.5%), 0.07 (0.017–0.188)), north east (NE) (n = 51 (9.8%), 0.143 (0.103–0.227)), north west (NW) (n = 50 (9.6%), 0.189 (0.138–0.295)), south east (SE) (n = 53 (10.2%), 0.012 (0.005–0.035)), south south (SS) (n = 100 (19.3%), 0.215 (0.076–0.379)) and south west (SW) (n = 164 (13.6%), 0.018 (0.007–0.036)). Kruskal–Wallis rank sum test displayed differences in the median values (p < 0.0001) of IgG level across the regions. The Pairwise comparison of the medians using Wilcoxon rank sum test revealed significant differences between (NC – NE, p = 0.00069), (NC – NW, p < 0.0001), (NC – SE, p < 0.0001), (NC – SS, p < 0.0001), (NC – SW, p < 0.0001), (NE – SE, p < 0.0001), (NE – SW, p < 0.0001), (NW – SE, p < 0.0001), (NW – SW, p < 0.0001), (SE – SS, p < 0.0001) and (SS – SW, p < 0.0001). There were not statistically significant differences between (NE – NW, p = 0.1), (NE – SS, p = 0.065), (NW – SS, p = 0.5) and (SE – SW, p = 0.18).

Figure 5.

Comparison of the causative pathogens in nine positive CPA-confirmed patients.

Figure 6.

Distribution of positive Aspergillus-specific IgG level. Radiological confirmed positive IgG cases of multidrug-resistant (MDR) TB patients (n = 22, min = 0.822, max = 2.499, median = 1.718, IQR = (0.898–2.095)), radiological confirmed positive IgG cases that was previously published (PPC) ([n = 17, min = 0.828, max = 3.034, median = 1.896, IQR = (1.109–2.464)), radiologically unconfirmed positive IgG cases of healthy blood donors (HBD) = (n = 14, min = 0.82, max = 0.324, median = 1.588, IQR = (1.238–2.205)).

Figure 7.

Receiving operating characteristics curve (ROC), AUC = 0.986, sensitivity = 100% (95% CI: 91.0–100%), specificity = 96.9% (95% CI: 95.0–98.2%) and accuracy = 97.1% (95% CI: 95.4–98.4%), positive predictive value (PPV) = 71%, negative predictive value (NPV) = 100%.