| ||1.4.1 Rapid Diagnostic Test (ICT platform) |
Most of our products are Rapid Diagnostic Tests (RDT), which uses solid phase immunochromatographic test (ICT) platform. There are two types of ICT - lateral flow and flow through. Our RDT is using the lateral flow ICT. A common lateral flow ICT has the following components:-
All these components are layered on to the plastic backing card. Each component is carefully positioned so that they overlap (Figure 1.4.1a). This allows the sample, buffer and reagents to flow from sample pad / conjugate pad through the nitrocellulose membrane to the adsorbent pad on the opposite end. The assembled strips are placed in a housing or cassette. The completed medical devices are packed and sealed in aluminium pouch to protect from moisture and light source. The packed medical devices are stable for almost two years when kept in room temperature.
- Sample Pad
- Conjugate Pad - The conjugate pad contains secondary antibodies which are conjugated with gold colloid.
- Solid-phase Membrane (nitrocellulose membrane) - The nitrocellulose membrane contains antigens and antibodies which are immobilized on the membrane. These antigens/antibodies form the test and control lines.
- Absorbent Pad
- Plastic tab
Patient sample (serum/plasma/whole blood) is drop on the sample pad (Figure 1.4.1b), for whole blood sample, buffer is dropped on the sample pad following patient sample.
The absorbent pad at the opposite end causes capillary action, which draw the sample (and buffer) toward the filter (Figure 1.4.1c). When the sample flows through the filter, red blood cells in the whole blood sample are held at the filter, while serum containing patient antibodies passes the filter toward the conjugate pad.
The conjugate pad contains secondary antibodies which are conjugated with gold colloid. When serum in the sample flows into the conjugate pad, patient antibodies in the serum bind to the gold colloid conjugated secondary antibodies, forming antibodies complexes (Figure 1.4.1d). The serum also solubilises the gold colloid conjugated secondary antibodies and the antibodies complexes flow toward the nitrocellulose membrane.
On the nitrocellulose membrane, antigens, which are specific to the disease, are immobilized on the test line, while antibodies, which are specific to the secondary antibodies, are immobilized on the control line. If the patient is having the disease of interest, the patient's sample will contain the disease specific antibodies (which are bound to the gold colloid conjugated secondary antibodies when they pass the conjugate pad). When the patient's sample flows through the test line, the disease specific antibodies in the sample will bind to the disease specific antigens immobilized on nitrocellulose membrane (Figure 1.4.1e).
Secondary antibodies which are not bound to the patient antibodies, bind to antibodies which are immobilized on the control line. Excess sample (and buffer), along with any unbound antibodies and unbound antibodies complexes flow to the absorbent pad.
Due to the secondary antibodies is conjugated with gold colloids which are maroon in colour, a maroon colour lines formed where the secondary antibodies bind. Therefore, for positive results, both test and control lines are visible with maroon colour line (Figure 1.4.1h). For negative results, since the patient does not have the disease of interest, the sample will not contain disease specific antibodies. Hence, there are no disease specific antibodies (which are bound to gold colloid conjugated secondary antibodies) to bind to the disease specific antigen immobilized on the test line and the test line does not appeared in maroon colour. In this case, only a control line is visible with maroon colour.
The control line serves as an internal quality control to ensure that the medical device is working properly. In the condition where control line does not appear, the result is considered invalid and the test is to be repeated with a new device.
Reszon Diagnostics International in vitro diagnostic medical devices which uses ICT platform are:-
- Typhidot Rapid IgM (also known as Typhi Rapid) for detection of Typhoid Fever (caused by Salmonella typhi)
- PanLF Rapid for detection of Lymphatic Filariasis (caused by Wuchereria bancrofti, Brugia malayi and Brugia timori)
- Brugia Rapid for detection of Brugian Filariasis (caused by Brugia malayi and Brugia timori)