Ground breaking Grewall

Diagnosis: To identify a disease or condition based on the signs and symptoms. Diagnosis is based on the the physical examination and tests such as blood tests, imaging (X-rays, CT scans) or biopsies.

Diagnostic test: A test to understand the condition or disease in order to plan the treatment (prognosis).
Biomarkers: Biological molecules found in the body or tissues that can be used as indicators for certain conditions in a diagnostic test. In a round-about way, a diagnostic test identifies the presence of a biomarker.

A rapid diagnostic test, as the name suggests, provides accurate result faster, usually within 20 minutes. Ideally, they should be easy to use by the consumer, compact and fast. For example, pregnancy kits.
Pregnancy kits use antibodies against the human chorionic gonadotrophin (hCG). hCG is a hormone secreted by pregnant women and it is used as a biomarker.

I have mentioned about the antibody-antigen reaction previously. Simply put, any antibody produced by the body is specific for the antigen (any foreign protein, like proteins present on the cell membrane of pathogens). The primary antibody is specific for the antigen. Next we use a secondary antibody (anti-antibody) which will be specific for the primary antibody. The secondary antibody contains a fluorophore or a chemical that produces color visible to us which is used to detect a positive result. Here’s a picture for clarity-

Similarly, RDTs for malaria, specific proteins of the malaria parasite but in some cases, instead of measuring the pathogen’s proteins, the antibodies made against the infection are detected.
For every pathogen, specific antibodies are secreted by the host to tackle the infection. Since the antibodies (IgG or IgM) are specific, they can also be used as a biomarker. In case of antibody-based RDTs, there are some offered to test for hepatitis, SARS-CoV-2 (COVID-19) and HIV infections. The fastest way to test for these is through blood samples.

History

Alfred and Helen Free developed the first ever diagnostic test strip. This Miles Laboratories research team used this technology to detect glucose in urine (to test for diabetes). Their technology used filter paper dipped in solutions and dried which could be used for testing. A positive test would be indicated by a change in color [1].

This was more of a qualitative approach, not a quantitative one. One of the determining factors of an ideal RDT is also the ease of transportation and storage. For example, an RDT test for malaria, a disease endemic to areas which do not have easy access to good medical facilities, needs to be easy to transport or carry without the need of say, maintaining a cold chain. The confirmatory test would be a blood test where one could identify the Plasmodium species but an RDT sure would be helpful.

How they work

Specificity and Sensitivity

These are the two factors that determine if an RDT can actually work. An easy way to remember would be-
Specificity: How accurately the antibody binds to the target protein.
Sensitivity: How much of the target protein the antibody is able to detect.

In specificity, the antibody should bind only to the target protein and not to any other similar protein. In that way, it is exclusive or specific to that target. Then, we would consider it highly specific.
In sensitivity, the antibody should be able to find even trace amounts or as little as possible. Then, we would consider it highly sensitive.

For example, the rapid antigen test for SAR-CoV-2 has shown a specificity of 99.9% with an overall sensitivity of 65.3% and in asymptomatic individuals sensitivity up to 44% [3].
For malaria, in case of a Plasmodium falciparumi, the sensitivity was around 99.7% and 95.5% for non-falciparum with a specificity of 99.5% [4]. To put it in perspective, anything above a 50% is good but obviously, the higher the better. It can never be a 100% because just a log fold increase in detection (able to detect pathogen if it reduced from 10^3 to 10^2) will bring the sensitivity from say, 99.9% to 99.99% and another increase will make it 99.999%. I will probably need another blog to help understanding why liquid hand washes can never provide us with a 100% and are still varying around the 99s (haha).

REFERENCES:

1. Ginsberg J. (2010) A National Historic Chemical Landmark: The Development of Diagnostic Test Strips. American Chemical Society.
2. World Health Organization (n.d.). How malaria RDTs work. Global Malaria Programme. https://www.who.int/teams/global-malaria-programme/case-management/diagnosis/rapid-diagnostic-tests/how-malaria-rdts-work (Accessed on April 18, 2024)
3. Jegerlehner, S., Suter-Riniker, F., Jent, P., Bittel, P., & Nagler, M. (2021). Diagnostic accuracy of a SARS-CoV-2 rapid antigen test in real-life clinical settings. International Journal of Infectious Diseases, 109, 118-122. DOI: 10.1016/j.ijid.2021.07.010
4. Berzosa, P., De Lucio, A., Romay-Barja, M., Herrador, Z., González, V., García, L., … & Benito, A. (2018). Comparison of three diagnostic methods (microscopy, RDT, and PCR) for the detection of malaria parasites in representative samples from Equatorial Guinea. Malaria journal, 17, 1-12. DOI: 10.1186/s12936-018-2481-4