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

Immunochromatographic test principle, procedure, working illustration

Introduction of Immunochomatographic Test (ICT)

Immunochomatographic test is the most  common test in diagnostic laboratory all over the world for the rapid finding in the field of diagnosis and treatment of  the 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 sample (matrix) without the need for specialized and costly equipment.

Principle of immunochromatographic  test

Immunochromatographic test principle  is the same as sandwich ELISA method. The only difference is in that immunological reaction is carried out on the chromatographic paper by capillary action. For this system, two kinds of specific antibodies against antigen are used. One of the antibodies is immobilized on the chromatographic paper while another  is labeled with colloidal gold and infiltrated into sample pad . An imunochromatographic unit is completed by attaching the sample pad at the end of the membrane. 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.  Appearance of red purple line on the membrane indicates the presence of antigen of interest in the sample . Liquid of the sample migrates through the membrane very fast, it 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’s 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 immunochrmatographic tests in which whole blood is applicable e.g. Malaria rapid diagnostic test in diagnosis of Malaria for detection Plasmodium species (vivax or falciparum) antigens.
Take an alcohol swab and disinfect the site of prick.Take a pricker and puncture the area below finger tip. Ooze the blood. In case of where there is need of serum or plasma, follow this procedure of blood collection. Clean venipuncture site with chlorhexidine or povidive iodine using back and forth friction scrub for 30 seconds. Allow it to dry for 30 seconds.Do not retouch site; if you must repalpate for vein, the area must be recleaned using above
procedure. Perform venipuncture. When required amount of blood is received, remove needle from skin. Hold pressure to site with gauze and apply adhesive bandage when bleeding stops. Inject blood specimens into  red protected vial for serum or EDTA purple vial for plasma. After clotting the blood, centrifuge the vial in case of serum but no waiting for clotting in case of plasma separation. Discard needle and syringe into a sharp container. Take two drops (volume depends on the nature of test and manufacturer guidelines) with sucker and dispensed in sample hole of strip. Add buffer to sample. Wait for 10-15 minutes. Observe result and interpret accordingly.

Result interpretation of Immunochromatographic test

Test negative: Only on band at control region

Test positive : Both bands at test and control regions

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

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

Advantage  and  disadvantage  of  Immunochromatographic test

Immunochromatographic tests  are a promising tool for reliable level of diagnostic performance.
Simple. Rapid assays that 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  the conditions and they are-

  • Helicobacter pylori test
  • Scrub typhus test
  • Dengue test

  • K39 test for visceral leishmaniasis (Kala-azar)

  • Malarial parasite test
  • Leptospira test
  • HAV test
  • HEV test
  • TORCH test,etc.
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