CFR (21 CFR part 610.10.e) clearly states that reactive unit should be further tested whenever a supplemental (additional, more specific) test has been approved for such use by the FDA.

HTLV Confirmation Test Kit

The MP Diagnostics HTLV Blot 2.4 is a qualitative enzyme immunoassay for the in vitro detection of antibodies to HTLV-I and HTLV-II in human serum or plasma. It is intended for use as a more specific supplemental test on human serum or plasma specimens found repeatedly reactive using screening procedures like the Enzyme-Linked Immunosorbent Assays (ELISA).

Chemical & Biological Principles of the Procedure
The HTLV Blot 2.4 nitrocellulo se strips are incorporated with HTLV-I and HTLV-II genetically engineered proteins and HTLV-I viral proteins derived from native inactivated disrupted viral particles.
It has improved sensitivity and specificity for both the confirmation and differentiation of HTLV-I and HTLV-II seroreactivities. This is accomplished by incorporating MTA-1, a unique HTLV-I envelope recombinant protein (rgp46-I), K55, a unique HTLV-II envelope recombinant protein (rgp46-II) and GD21, a common yet specific HTLV-I and HTLV-II epitope envelope recombinant protein (rgp21).
Each nitro celluiose strip also includes an internal sample addition control to minimize the risk of false negatives due to operational errors.
Individual nitrocellulose strips are incubated with diluted serum or plasma specimens and controls.
Specific antibodies to HTLV-I/II, if present in the specimen will bind to the HTLV-I/II proteins on the strips.
The strips are washed to remove unbound materials while antibodies that bind specifically to the HTLV proteins can be visualized using a series of reactions with goat anti-human IgG conjugated with alkaline phosphatase and the substrate, BCIP/NBT.
Of the proteins applied to the nitrocellulose strips, five are generally used to confirm the presence of antibodies against HTLV-I/II. These are the rgp46-I, rgp46-II, GD21, p19 and p24.Type-specific recombinant envelope protein rgp46-I is specific for HTLV-I, while rgp46-II is specific for HTLV-II; these antigens are used to differentiate between HTLV-I and HTLV-II infections.
GD21, a third recombinant envelope protein, is broadly immunoreactive with sera or plasma from HTLV-I and HTLV-II infected individuals. Two GAG proteins, p19 and p24, which are reactive to HTLV-I and cross-reactive to HTLVII, are used to confirm the presence of antibodies. It has been found that reactivity against p19 was greater than or equal to that against p24 in subjects who had HTLV-I infection confirmed by PCR. Conversely, p24 bands were stronger than p19 bands in persons who had PCR-confirmed HTLV-II infection. That is, if p19 is greater than or equal to p24, HTLV-I infection is suggested, and if p24 is greater than or equal to p19, HTLV II infection is suggested.
Disclaimer Notice
The health information contained herein is provided for educational, reference purposes only, and is not intended to substitute professional medical advice, diagnosis, or treatment with a healthcare provider. All decisions regarding patient care must be made with a healthcare provider, considering the unique characteristics of the patient. Please check the availability and relevance of the information in your respective country.