This reason is deep and professional, involving the research field. If we discuss the principle directly, non-professionals may not understand it. Simply put, rheumatic factors in the blood of patients with rheumatism will react with the reagents for detecting syphilis, resulting in false positives.

The principles of these two tests are listed as follows for reference only:

1, the principle of syphilis reagent card:

special technology

Syphilis antibody detection reagent is prepared by using the principle of double antigen sandwich method and colloidal gold as indicator.

Add one drop of serum to be tested and two drops of buffer solution to the sample hole of the test card. If the serum contains a certain amount of Syp-Ab, it will combine with Syp-Ag in the gold label to form a complex. When the chromatography reaches the T-line, the complex immunobinds to the recombinant antigen of syphilis embedded in the T-line, so that the bridged colloidal gold is colored on the T-line. When the remaining gold label continues to be chromatographed to the C line, the gold label and the polyclonal antibody embedded here are immunobound to bridge the colloidal gold and develop color. If the serum does not contain Syp-Ab or less than a certain amount, the T-line malaria recombinant antigen will not react with the gold label, and the C-line polyclonal antibody will still bind with the gold label, showing a single color.

Result interpretation

Positive: when both T-ray and C-ray are red bands, the test result is positive;

Negative: T-ray is not colored, and C-ray is negative when colored with red band;

Invalid: the C line is not colored, indicating that the test failed or failed.

2, the principle of rheumatoid factor reagent:

Rheumatoid factor (RF) exists in the serum of patients with rheumatoid arthritis (RA). It is an autoantibody with denatured IgG as the target antigen, which mainly exists in the serum and joint fluid of patients with rheumatoid arthritis. It is an antibody against denatured IgG and belongs to IgM type. It can bind to IgGFc fragment. There are RF-producing B cell clones in RA patients and about 50% healthy people. Under the direct action of denatured IgG (or IgG bound to antigen) or EB virus, RF can be synthesized in large quantities. There are few cell clones that produce RF in healthy people, and soluble factors secreted by monocytes can inhibit the production of RF, which is generally difficult to detect. RF is mainly IgM autoantibodies, but there are also IgG, IgA, IgD and IgE. The clinical significance of various radio frequencies is different. The determination of IgG, IgA, IgM-like RF usually adopts indirect ELISA method, that is, heat agglutinating rabbit IgG is used to coat the micropores of the reaction plate, the sample is added, and then anti-human IgG, IgA, IgM enzyme-labeled antibodies are added respectively, so that the reaction will develop color with the substrate. In order to prevent RF interference of various Ig classes, the labeled antibody F(ab)2 fragment was used as the enzyme-labeled antibody.

principle

Denatured IgG was coated on polystyrene latex particles. When this sensitized latex meets RF in the serum to be tested, it will agglutination with naked eyes, which is called latex agglutination test.

In the micropores of polystyrene reaction plate coated with heat agglutinating denatured IgG, the serum to be detected is added, and if RF exists, it is bound to each other, then the enzyme-labeled heat agglutinating denatured IgG is added to react with it, and the color can be developed after adding the substrate. According to the degree of color development, the existence and level of RF can be judged. This is a double antigen sandwich ELISA.

Latex agglutination test

RF is an anti-human denatured IgG antibody mainly existing in patients with rheumatoid arthritis, which can bind to Fc segment of IgG. Denatured IgG is coated on polystyrene latex particles, and when sensitized latex meets RF in serum to be tested, it can agglutinate with naked eyes.

operate

Inactivate at 1 and 56℃ for 30 minutes (inactivate C 1q to prevent false positive agglutination). The serum to be tested was diluted with 0. 1 mol/L glycine buffered saline (pH 8.2) to 1: 20 (0.05ml serum was added to 1ml physiological saline).

2. Take 1 drop (about 0.05ml) of this diluted serum, add 1 drop latex RF reagent to the square of black square glass, immediately shake the reaction plate for 3 minutes to make it fully mixed, and then observe it under direct vision. Positive and negative controls were set for each experiment.

reference value

Normal people 1: 20 diluted serum was negative.

Result judgment

Most of the normal latex agglutination tests are negative, and those with obvious agglutination within 3 minutes are positive. Positive samples should be diluted twice to determine the titer of serum. According to the degree of color development, ELISA can be compared with positive and negative controls to make a positive or negative judgment.

3. Analysis of the influence of rheumatoid factor on syphilis antibody detection. If you are interested, you can have a look at this document. Attached with screenshot.