1.Agglutination card
- It is an agglutination card containing circles.
- Each circle can be used for one agglutination reaction.
- Background colour of circle can be white or black-depending upon the colour of antigen of the test, appropriate colour card is used eg. Antigen in RPR test is black, so white card is used to make the agglutinate clearly visible.
Use:
- It is used to carry out agglutination test for detection of antibodies against various antigens or vice versa
- Test is employed for detection of RA factor, ASO titer, Widal test for Typhoid, CRP detection, RPR test for Syphilis.
- It is also important for detection of antigens like Cryptococcal antigen, pneumococcal antigen in CSF.
2.RPR
- RPR slide flocculation test using disposable plastic cards having clearly defined circles.
- It is similar to VDRL test with some differences such as:
- RPR antigen has a prolonged shelf-life, therefore it is preferred to test individual sample (less sample load) where as VDRL is preferred when samples are tested in batches (large sample load)
- Results can be read in naked eyes, without the need of a microscope, as the clumps formed are bigger in size
- It can only be used for detecting antibodies in blood;not in CSF
- It is more expensive than VDRL.
3.Immunochromatographic test (ICT)
This test is based on lateral flow technique. It is widely used in diagnostic laboratories because of its simplicity, low-cost and rapidity. It can be used for both antigen and antibody detection in sample. Principle of antigen detection method is described below.
Principle of ICT (Antigen Detection)
The test system consists of a nitrocellulose membrane(NCM) and an absorbent pad. Two formats are available:cassette or strip. The NCM is coated at two places in the form of lines—a test line, coated with monoclonal antibody targeted against the test antigen and a control line, coated with anti-species immunoglobulin.Specific Ab against the target Ag labelled with chromogenic marker (specific Ab tagged with colloidal gold or silver, a visually detectable marker) is infiltrated in the absorbent
pad lining the sample window. The sample (serum) containing the test antigen is added to sample well; it reacts with antibody labeled with chromogenic marker (colloidal gold or silver, a visually detectable marker) Both ‘Ag-specific Ab-colloidal gold complex’ as well as the ‘free colloidal gold labeled Ab’ move laterally along the nitrocellulose membrane
Test band: At the test line, the Ag-labeled Ab complex is immobilized by binding to the monoclonal Ab in the test line to form a colored band
Control band: The free colloidal gold labeled Ab can move further and binds to the anti-human Ig to form a color control band. If the control band is not formed, then the test is considered invalid irrespective of whether the test band is formed or not.
4.Flow-through Assay
Flow-through tests are another type of rapid diagnostic assays which differ from ICT in two aspects:
(1) protein A is used for labeling antibody instead of gold conjugate and
(2) the sample flows vertically through the nitrocellulose membrane (NCM) as compared to lateral flow in ICT.
Flow-through tests can be used for both antigen and antibody detection.
HIV TRIDOT test is a classical example. It detects antibodies to HIV-1 and 2 separatelyin patient’s serum.
The test system is in a cassette format, consisting of a NCM and an absorbent pad.
The NCM is coated at three regions- two test regions coated with HIV-1 and 2 antigens and a third control region coated with antihuman Ig. Sample and buffer reagents are added sequentially from the top following which they pass through the membrane and excess fluid is absorbed into the underlying absorbent pad .
As the patient’s sample passes through the membrane, HIV antibodies, if present bind to the immobilized antigens
Test dots: Protein-A conjugate (present in buffer) binds to the Fc portion of the HIV antibodies to give distinct pinkish purple
DOT(s), separately for HIV-1 and 2 antibodies
Control dot: Irrespective of whether the HIV antibodies are present or not, protein-A can bind to any IgG present in serum and the IgG-protein A complex can further bind to the antihuman Ig at the control line to give a pinkish purple DOT.