Correlation of Automated Chemiluminescent Method with Enzyme-Linked Immunosorbent Assay (ELISA) Antibody Titers in Convalescent COVID-19 Plasma Samples: Development of Rapid, Cost-Effective Semi-Quantitative Diagnostic Methods

Figures & data

Table 1 Characteristics of Volunteer Donors and Their Donated Plasma

Characteristics	Median (IQR)/N (%)
Age, years	39.9 (30.3–49.7)
Antibody test prior to donation
Performed	91 (50.8)
Not performed	88 (49.2)
Antibody test result prior to donation (S/Co)	2.3 (1.7–3.1)
CCP chemiluminescent results
0.02 to 1	21 (11.7)
>1-5	71 (39.1)
>5-10	32 (17.9)
>10-20	27 (15.1)
>20	29 (16.2)
Donated plasma ELISA titers
Negative	15 (8.4)
1:32	2 (1.1)
1:64	7 (3.9)
1:128	10 (5.6)
1:256	19 (10.6)
1:512	31(17.3)
1:1024	35 (19.6)
1:2048	22 (12.3)
1:4096	22 (12.3)
1:8192	5 (2.8)
1:16,384	8 (4.5)
1:32,768	3 (1.9)

Table 2 Sensitivity of the Antibody Tests Compared to SARS-CoV-2 Viral RNA RT PCR

	Chemiluminescent Assay	ELISA
Sensitivity	88.3%	90.5%
No.	158/179	162/179
95% CI	82.5–91.8%	83.6–92.6%

Table 3 CLIA S/Co Values and ELISA Titers of Donated Convalescent Plasma

CLIA S/Co Values	N (%)	ELISA IgG Titer Equivalent
0.02 to 1	21 (11.7)	Neg to 1:512
>1 to 5	70 (39.1)	Neg to 1:4096
>5 to 10	32 (17.9)	Neg to 1:4096
>10 to 20	27 (15.1)	1:128 to 1:4096
>20	29 (16.2)	1:2048 to 1:32,768
ELISA IgG Titer	N (%)	CLIA S/Co Value Equivalent
Neg	15 (8.4)	0.02 to 6.25
1:32	2 (1.1)	0.37 to 0.5
1:64	7 (3.9)	0.09 to 9.71
1:128	10 (5.6)	0.25 to 11.51
1:256	19 (10.6)	0.05 to 7.66
1:512	31 (17.3)	0.81 to 19.65
1:1024	35 (19.6)	1.2 to 15.52
1:2048	22 (12.3)	2.32 to 26.62
1:4096	22 (12.3)	1.92 to 44.12
1:8192	5 (2.8)	23.48 to 34.26
1:16,384	8 (4.5)	28.08 to 63.4
1:32,768	3 (1.9)	28.82 to 52.64

Abbreviations: CLIA, chemiluminescent immunoassay, S/Co, sample to control intensity ratio, ELISA, enzyme-linked immunosorbent assay, Neg, negative, “0.

Table 4 Linear Regression Model of the Root of Intensity as a Predictor of the Logarithmic Value of ELISA IgG Titer

Parameter Estimates
Variable	DF	Parameter	Standard	t Value	Pr > |t|	95% Confidence Limits
		Estimate	Error
Intercept	1	4.89496	0.12611	38.82	<0.0001	4.64593	5.14399
Root of CLIA S/Co	1	0.70612	0.03826	18.45	<0.0001	0.63056	0.78168

Figure 1 Linear regression analysis on the relationship of CLIA S/Co values and ELISA IgG titers.

Table 5 Representative CLIA Intensity to Expected Titer Concentrations

CLIA S/Co Values	Estimated ELISA IgG Antibody Titer
	Estimated ELISA IgG Antibody Titer, Mean	Lower 95%	Upper 95%
0	1:133.6	1:104.2	1:171.4
1	1:270.7	1:195.7	1:374.5
2	1:362.7	1:254.1	1:517.7
3	1:454.0	1:310.5	1:663.7
4	1:548.5	1:367.6	1:818.4
5	1:648.0	1:426.6	1:984.2
6	1:753.4	1:488.1	1:1162.9
7	1:865.4	1:552.4	1:1355.8
8	1:984.5	1:619.8	1:1563.9
9	1:1111.3	1:690.6	1:1788.3
10	1:1246.3	1:765.0	1:2030.2
15	1:2058.5	1:1197.6	1:3538.4
20	1:3142.7	1:1747.4	1:5652.0
25	1:4562.2	1:2437.5	1:8538.9
30	1:6390.3	1:3293.3	1:12,399.7
35	1:8711.7	1:4343.2	1:17,474.0
50	1:19,692.3	1:8997.1	1:43,101.0
60	1:31,714.9	1:13,769.7	1:73,046.9
70	1:49,157.5	1:20,365.0	1:118,657.2

Abbreviations: CLIA, chemiluminescent immunoassay, S/Co, sample to control intensity ratio, ELISA, enzyme-linked immunosorbent assay.

Figure 2 Receiver operating characteristic (ROC) curve showing an area under the curve (AUC) of 0.9143. The orange line denotes a CLIA S/Co cutoff value of 8.2.