Immunochromatographic Test : Introduction, Principle, Procedure and resul

Immunochromatographic Test (ICT): Introduction, Principle, Procedure and Result interpretation

Immunochromatographic test principle, procedure, working illustration

Introduction of Immunochromatographic Test (ICT)

The immunochromatographic test is the most common test in diagnostic laboratories all over the world for rapid finding in the field of diagnosis and treatment of patients. Lateral flow tests also known as lateral flow immunochromatographic assays, are simple test devices intended to detect the presence (or absence) of a target analyte (antigen or antibody) in the sample (matrix) without the need for specialized and costly equipment.

Principle of immunochromatographic test

The immunochromatographic test principle is the same as the sandwich ELISA method. The only difference is that immunological reaction is carried out on the chromatographic paper by capillary action. For this system, two kinds of specific antibodies against the antigen are used. One of the antibodies is immobilized on the chromatographic paper while another is labeled with colloidal gold and infiltrated into a sample pad. An immunochromatographic unit is completed by attaching the sample pad at the end of the membrane. The liquid sample is dropped on the sample pad. The antigen in the sample forms an immunocomplex with the antibody labeled with colloidal gold. Its complex moves along with the liquid sample and makes a contact with the antibody immobilized on the membrane. It is followed by forming an immuno- complex with the immobilized antibody. It results in generating a colored red-purple line. The appearance of a red-purple line on the membrane indicates the presence of an antigen of interest in the sample. The liquid of the sample migrates through the membrane very fast, which makes it possible to detect the presence or absence of antigen within 15 minutes as shown above image.

Components of Immunochromatographic test device

Sample pad: It acts as a sponge and holds an excess of sample fluid. Once soaked, the fluid migrates to the second element.
Conjugate pad: A dried format of bio-active particles in a salt-sugar matrix that contains everything to guarantee an optimized chemical reaction between the target molecule (e.g. an antigen) and its chemical partner (e.g., antibody) that has been immobilized on the particle surface. Control: It contains an antibody that picks up free latex/gold in order to confirm the test has operated correctly. Test: It contains a specific capture molecule and only captures those particles onto which an analyte (antigen or antibody ) molecule has been immobilized.

Procedure of Immunochromatographic test

Note: For those immunochromatographic tests in which whole blood is applicable e.g. Malaria rapid diagnostic test in the diagnosis of Malaria for detection of Plasmodium species (vivax or falciparum) antigens.
Take an alcohol swab and disinfect the site of the prick. Take a pricker and puncture the area below the fingertip. Ooze the blood. In the case where there is a need for serum or plasma, follow this procedure of blood collection. Clean the venipuncture site with chlorhexidine or povidone-iodine using a back-and-forth friction scrub for 30 seconds. Allow it to dry for 30 seconds. Do not retouch the site; if you must repalpate for vein, the area must be recleaned using the above procedure. Perform venipuncture. When the required amount of blood is received, remove the needle from the skin. Hold pressure on to the site with gauze and apply an adhesive bandage when the bleeding stops. Inject blood specimens into a red-protected vial for serum or EDTA purple vial for plasma. After clotting the blood, centrifuge the vial in case of serum but not waiting for clotting in case of plasma separation. Discard the needle and syringe into a sharp container. Take two drops (the volume depends on the nature of the test and manufacturer guidelines) with a sucker and dispensed them in the sample hole of the strip. Add buffer to the sample. Wait for 10-15 minutes. Observe results and interpret accordingly.

Result interpretation of Immunochromatographic test

Test negative: Only on the band at the control region

Test positive: Both bands at test and control regions

Test invalid: No band at all or band only at the test region

Note: The test is positive as shown above image because of having both bands at the control and test regions or lines.

Advantages and disadvantages of Immunochromatographic test

Immunochromatographic tests are a promising tool for a reliable level of diagnostic performance.
Simple. Rapid assays can be completed in 15-20 minutes. They reduce the need for trained examiners and costly equipment. It can be used under harsh field conditions. ICT assays are not as sensitive as other immunoassays like ELISA or RIA. The results of this test should be confirmed by other reference tests.

Application of Immunochromatographic test

This test is widely adopted as the quick or spot or screening test for laboratory diagnosis of various diseases etiological agents as well as conditions and they are-