【Rapid Antigen Test】What is Ct value of rapid test?



Due to the surging demands for rapid antigen tests, the community has been awash with different kinds of rapid testing products. Medical professionals have advised to choose rapid tests based on their sensitivity, specificity and Ct value, but what are these values exactly? How do we choose a rapid test that can accurately show our infection status? INDICAID is here to break down all these parameters with this article.

What are “Sensitivity” and “Specificity”?

It is recommended by the government to check the “sensitivity” and “specificity” of a rapid testing product before making your purchase, in order to acquire a more effective and accurate result, and to avoid “false positive” or “false negative” results.

  • Sensitivity: True positive rate, the proportion of detecting positive results from genuinely positive samples
  • Specificity: True negative rate, the proportion of detecting negative results from genuinely negative samples

According to the WHO interim guidance, rapid antigen tests are recommended to meet the minimum performance of at least 80% sensitivity and 97% specificity, meaning at least 8 positive cases can be identified out of 10 positive samples; while at least 97 negative cases are detected from 100 negative samples (a maximum of 3 false positive cases). Therefore, it is important for users to understand if the products have met the requirements of WHO in order to obtain more accurate test results, as well as safeguarding you and your family’s health.

What Is “Ct Value”?

Ct Value (Cycle Threshold Value) is a relative measure of the concentration of a target gene present in a sample. Due to the low amount of virus RNA collected in a sample, RT-PCR technique is usually adopted to reverse the transcription of RNA into DNA and hence amplifying specific DNA targets. To put it simply, Ct value increases by one unit when undergoing one amplification, and hence Ct value can be understood as the number of cycles of amplification required for a virus to be detected.

For instance: Ct Value = 30 implies the target gene in the sample is detected after being multiplied by 230.

Ct ValueViral Load
Relatively higher
25 – 30Relatively lower
>30Very low

To protect yourself and your family, and avoid further spreading viruses in the local community, it is suggested to purchase rapid antigen tests that can provide test results accurately even when the viral load is relatively lower in the sample. If there are products claiming to reach 100% sensitivity without mentioning its sensitivity of test with Ct value >25, it might not be able to accurately detect an infected person with lower viral load, indicating a possible false negative case.

Serious consequences due to low accuracy, false positive or false negative results

False Positive (Mistaken as infected)False Negative (Mistaken as not infected)
Unnecessary quarantine
Increase in spread of the virus and may affect household members (especially children and elderly), colleagues and other close contacts
Unnecessary anxiety and panicDelay in seeking medical advice
Consuming time and resources for further testing

Based on our clinical analysis, INDICAID®INDICAID®Rapid Antigen Test can reach 100% sensitivity of test with both Ct value lower than 25 or between 25 and 30, which is a significant proof of INDICAID’s high accuracy and its ability to detect infected persons with a relatively lower viral load.

INDICAID Rapid Antigen Test - Overall Clinical Sensitivities At Different Ct Values

Ct ValueSensitivity
0 < Ct ≤ 25
25 < Ct ≤ 30100%
預約 INDICAID lab 中環私家檢測中心核酸檢測 (強制檢疫期間適用)

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How to check the qualification of a rapid test product?

Apart from understanding various accuracy values of a rapid test kit, one should also beware of the reliability of its manufacturer by looking into their relevant certification from authorities. According to the latest announcement from the Hong Kong government, the following websites can provide you with references of a rapid test product’s credibility:

  • List of free distribution of Rapid Antigen Test kits by the Government
  • Listed Rapid Antigen Tests under the Medical Device Administrative Control System
  • EU CE In Vitro Diagnostic Medical Devices
  • List of antigen diagnostic tests with Emergency Use Authorization