TABLE 1.

Main pattern of surrogate tests, including standard cut‐off (CO) and proposed FDA guidance for high‐titer donors

Test	Principle	Antigen(s)	Second stage	Antibodies	Standard cut‐off (CO)	FDA alternative CO (high‐titer)
c‐Pass	ELISA	Rec RBD (HRP)	Rec hACE2‐R (solid phase) + TMB	IgM, IgG	≥20%	≥68%
Ortho CoV‐2T	CLIA	Rec S1 (biotin)	Conjugated‐ recombinant S1 (HRP)	IgM, IgG, IgA	≥1.0	≥9.5
Elecsys anti‐ SARS‐CoV‐2S	E‐CLIA	Rec RBD (biotin); rec RBD (ruthenium); streptavidin‐coated magnetic particles	Binding to paramagnetic spheres + electric pulse (chemiluminescence)	IgG, IgM	≥ 0.8 U/ml	≥132 U/ml
Elecsys anti‐SARS‐CoV‐2	E‐CLIA	Rec N (biotin); rec NP (ruthenium); streptavidin‐coated magnetic particles	Binding to paramagnetic spheres + electric pulse (chemiluminescence)	IgG, IgM	≥1.0	≥109
ICB IgG anti‐NP	ELISA	nCoV‐PS‐Ag7	Conjugated goat anti‐human IgG (HRP)	IgG	S/CO a  = 1.0	S/CO ≥5.0 b
ICB IgM anti‐NP	ELISA	nCoV‐PS‐Ag7	Conjugated goat anti‐human IgM (HRP)	IgM	S/CO a  = 1.0	ND
ICB IgA anti‐NP	ELISA	nCoV‐PS‐Ag7	Conjugated goat anti‐human IgA (HRP)	IgA	S/CO a  = 1.0	ND

Abbreviation: ND, Not done.

^a Presented as signal/cut‐off (S/CO) ratio.

^b High S/CO based on previous Reference 1.