Rapid Diagnostic Test Kits: How do they work?

Rapid diagnostic test kits are devices that are used at the point of care to detect disease or confirm symptoms through the principle of lateral flow immunochromatography. They have antigens or antibodies embedded in them. The capture antibodies/ antigens are tagged with coloring agents (a dye) that form a distinct color when it interacts with the formed antibody/ antigen complex.

Because of their simplicity, these devices are very valuable in places where there are limited resources, or in case of an emergency where diagnostic means such as polymerase chain reactions or ELISA tests are least helpful in saving a patient’s life.

The various types of lateral flow tests include strips, cassettes, multi-devices, and midstream devices.

The components of a rapid diagnostic test kit include:

1. PVC Plate

This is plastic casing or container that holds the sample and conjugate pads, the absorbent wick and the analytical membrane. Some test kits have the plate and casing while others do not have.

2 Conjugate Pad

This is the smallest part of the lateral flow test kit. it is responsible for absorbing the volume in which the detector conjugate is added to the pad. It does not interact with the conjugate, however, it ensures that the conjugate integrity is maintained upon drying. When it comes into contact with the sample fluid, it releases the conjugate easily and completely and thus allowing for the interaction between the detector reagents in the conjugate and the target analyte in the sample.

3 Sample Pad

This is the section of the device in which the sample is introduced for analysis or target antibody/antigen detection. its design considers certain characteristics of the sample being handled such as the target molecule concentration, sample composition in terms of factors such as the pH, sample viscosity, the need for particle retention and the unspecific interactions of the target with the test line reagents.

It is therefore of great importance to define the sample volume to be applied and select appropriate sample volume material because for instance, if the sample is blood and its more than the necessary volume, it could lead to red blood cell breakthrough problems; if it is hemolysed, the free hemoglobin could stain the background, making the result difficult to read.

4 Analytical Membrane

This is a large pore sized nitrocellulose membrane that is a varies t accommodate different sample types. It contains a surfactant that is usually anionic which makes them hydrophilic. These membranes do not have pores, they are however made up of nitrocellulose fibers.

5 The Wick/ Absorbent Pad

The wick as its name suggest is tasked with absorbing the sample liquid and the reagents that had not been used at the test and control lines. this is done to prevent the backflow of this liquid into the dry membrane as long as possible. the selection process of an absorbent pad includes considering one that can contain fluid higher than the sample volume for efficiency.

Working Mechanism of Rapid Diagnostic Test Kits

The sample is placed on the sample pad at the end of the strip alone as in the case of serum, urine or plasma or it may be mixed with a buffer for certain specific tests.

Upon addition of the sample, the detector molecules solubilize and mix with the analyte in the sample if present. they then move by capillary action along with the sample pad into the membrane.

The sample and detector molecules move towards the analyte capture molecules in the test line where they are immobilized as a thin stripe on the nitrocellulose membrane if the analyte is present. the excess buffer and sample are absorbed by the absorbent pad.

Lateral flow assay in rapid diagnostic test devices as illustrated by NASA