Discrepancies in COVID-19 PCR Results Among Family Members

Q: Discrepancies in COVID-19 PCR Results Among Family Members

Hello： Regarding your inquiry, I will explain it in three aspects： <br>1. <br>Positive and negative results of rapid tests and PCR： The sensitivity of both tests depends on the viral load and the sampling process (the latter has a lower probability). <br>Additionally, rapid tests

Why is it that some family members test positive for COVID-19 via PCR while others test negative? It's truly puzzling whether those who tested negative have recovered from the infection or if they never contracted the virus at all?

Hello Dr.
Lin, I would like to first describe my family members and the timeline of our COVID-19 infection.
We have Grandpa (66), Grandma (61), myself (41), my wife (40), our eldest daughter (10), son (8), youngest daughter (2), our foreign domestic helper (40), my sister (40), brother-in-law (44), my sister's eldest son (9), sister's daughter (5), and sister's youngest son (1.5).
Since we all live in the same house and have dinner together, on the evening of May 14, we received a call from the school informing us that a classmate had tested positive for COVID-19 via PCR.
All students were required to self-monitor.
That night, we started sleeping in separate rooms: Grandpa in one room, Grandma in another, myself and my son in one room, my wife and youngest daughter in another, our eldest daughter in one room, the foreign domestic helper in one room, my sister and her youngest son in one room, and my brother-in-law, sister's eldest son, and sister's daughter in one room, totaling eight rooms.
We also separated our meals.
On May 17, our eldest daughter tested negative on a rapid test, while our son tested positive.
That evening, we immediately took our son for a PCR test, and I got tested as well, with Grandma accompanying us and getting tested too.
The results showed that all three of us were positive, while the others continued to test negative daily.
We were really worried, so we arranged for another PCR test on May 19.
My wife tested positive, my sister tested negative, my sister's youngest son tested positive (as he sleeps with my sister), my brother-in-law tested positive, my sister's eldest son tested negative, and my sister's daughter tested negative.
On May 21, our eldest daughter and youngest daughter both tested negative (but the youngest daughter sleeps with my wife, who tested positive).
However, on May 20, we tried several brands of rapid tests (Roche, GenBody, HECIN), and those who previously tested positive turned negative, which made us very anxious.
Our eldest and youngest daughters may have previously been infected, but after so many days, their CT values might be above, for example, 34, leading to a determination of positive, or they may not have been infected at all (as uninfected individuals can receive the BNT vaccine immediately, while those who have been infected need to wait at least three months).
Since our entire family has tested positive (PCR positive), we have not experienced any symptoms like coughing or runny nose.
If we hadn’t gone for PCR testing, we wouldn’t have known we were infected.
I wonder if you could give me some advice...
Should we pay for a PCR test to check the CT values, or should we look for antibody tests (which are hard to find) for verification? If our eldest daughter has a high CT value post-infection, but we believe she has never been infected, we are concerned about administering the BNT pediatric vaccine to her.
Thank you, doctor.

Kochen, 40~49 year old female. Ask Date: 2022/05/23

Dr. Lin Zhebin reply Family Medicine

Hello: Regarding your inquiry, I will explain it in three aspects:
1.
Positive and negative results of rapid tests and PCR: The sensitivity of both tests depends on the viral load and the sampling process (the latter has a lower probability).
Additionally, rapid tests may have varying sensitivities depending on the manufacturer.
During the early stage of infection (approximately 3 days) and the later stage (around 7-10 days) when recovery is nearing, the viral load tends to be lower, which may result in a negative rapid test.
Although a negative rapid test does not rule out a diagnosis, it at least indicates a low viral load, which correlates with lower transmissibility.
PCR is the most accurate test for diagnosing COVID-19; unless the viral load is extremely low (during the incubation period), an infection should generally yield a positive result.
2.
Whether your older and younger daughters are infected: Given the confirmed cases in your household, it can be considered a high-prevalence area for COVID-19.
Although their previous PCR tests were negative (which could indicate an incubation period), I would lean towards the possibility that they are likely infected.
If they are currently not experiencing any significant discomfort, I personally believe there is no need to go to the hospital for PCR and antibody testing.
If they do experience any physical discomfort, they should seek medical evaluation and treatment.
Currently, COVID-19 infection has become a norm, and the focus of prevention should primarily be on reducing severe cases.
3.
Whether your older and younger daughters can receive the vaccine: In fact, the transmissibility of COVID-19 is very low 7-10 days after infection.
If their rapid tests are negative, it is safe for them to go out and get vaccinated.
There is no requirement to wait more than three months after infection to receive the vaccine.
I believe you meant that antibodies from an infection can last around 3-6 months, while antibodies generated from vaccination can last longer.
Therefore, I recommend that you have your older and younger daughters vaccinated as soon as possible to effectively reduce the risk of infection and avoid severe illness.

Reply Date: 2022/05/23

More Info

Understanding the discrepancies in COVID-19 PCR results among family members can be quite perplexing, especially in the context of a household where multiple individuals have been exposed to the virus. Based on the information you provided, several factors could contribute to the varying PCR results among your family members, and I will address these below.

1. Viral Load and Timing of Testing: One of the primary reasons for differing PCR results is the viral load present in each individual at the time of testing. The viral load can fluctuate significantly during the course of an infection, often peaking shortly after exposure and then declining as the immune response kicks in. If a family member is tested during a period of low viral load, they may receive a negative result even if they are infected. This is particularly relevant for your daughter, who tested negative on a rapid test but may have been in the early stages of infection when the viral load was not high enough to detect.

2. Sensitivity of Testing Methods: The sensitivity of PCR tests can vary based on the specific test used, the sample collection technique, and the timing of the test in relation to exposure or symptom onset. It’s important to note that PCR tests are generally more sensitive than rapid antigen tests, which means they can detect lower levels of the virus. However, if a rapid test is performed too early or too late in the infection cycle, it may yield a false negative result.

3. Cross-Contamination and Testing Errors: In some cases, discrepancies can arise from cross-contamination during sample collection or processing. Additionally, human error in administering the test or interpreting results can also lead to inconsistencies. It’s crucial to ensure that testing is conducted in a controlled environment to minimize these risks.

4. Previous Infection and Antibody Response: Your concern regarding your daughters' previous infections and their current PCR results is valid. If they had COVID-19 previously, they may have developed antibodies that could affect their immune response. However, the presence of antibodies does not guarantee immunity, and reinfection can occur. The CT (cycle threshold) value from PCR tests can indicate the viral load; a higher CT value (e.g., above 34) typically suggests a lower viral load, which may not be infectious. If your daughters have high CT values, it is possible they are no longer infectious.

5. Vaccination Considerations: Regarding vaccination, if your daughters have had a confirmed case of COVID-19, they should ideally wait at least three months before receiving a vaccine. However, if they are determined to be uninfected based on testing and clinical evaluation, they may be eligible for vaccination sooner. It is essential to consult with a healthcare provider to assess their vaccination status based on their testing history and potential exposure.

6. Follow-Up Testing: If you are concerned about the accuracy of the tests, it may be worthwhile to pursue additional PCR testing, especially if you are considering vaccination for your daughters. Testing for antibodies can also provide insight into past infections, but availability may vary. Discussing these options with a healthcare provider can help clarify the best course of action.

In summary, the discrepancies in PCR results among your family members can be attributed to factors such as timing of testing, viral load variations, test sensitivity, and previous infections. It is advisable to consult with a healthcare professional to determine the most appropriate next steps, including potential follow-up testing and vaccination considerations. Your proactive approach to understanding and addressing these issues is commendable, and it is crucial to prioritize the health and safety of your family during this ongoing pandemic.

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