The C line on a Covid test indicates whether the test is working properly or not. A negative or faint C line means the test did not work correctly and the results cannot be trusted. Here is a quick rundown of what the C line means on different types of Covid tests:
PCR Tests
PCR (polymerase chain reaction) tests look for genetic material of the Covid virus. These tests have a C line that acts as a control line. The C line uses antibodies that bind to a type of protein that is not related to Covid but is added to the test strip. If the C line appears, it means the test reagents are working properly. If the C line does not appear, the test is invalid regardless of the result of the test line.
What a negative or faint C line means on a PCR test:
- The reagents or test strip failed
- The sample was not processed correctly
- An error occurred while running the test
In any of these cases, a negative or faint C line means the test is invalid and the result cannot be used. A new sample should be collected and run again.
Rapid Antigen Tests
Rapid antigen tests look for proteins from the Covid virus. Like PCR tests, they also have a C line to act as a control. The C line binds to antibodies in the test strip to demonstrate that the reagents are working correctly.
What a negative or faint C line means on a rapid antigen test:
- The reagents in the test strip degraded
- The test was not performed properly
- There were errors in the manufacturing of the test
As with PCR tests, a negative or faint C line means the rapid antigen test is invalid. The result cannot be used and a new test should be taken to get an accurate result.
Antibody/Serology Tests
Antibody tests look for antibodies created by the immune system to fight off Covid infection. They function differently than PCR and rapid antigen tests. Antibody tests have a C line and a Covid line:
- C line: Binds to human antibodies to demonstrate the test is working correctly.
- Covid line: Binds to Covid-19 antibodies in the blood, if present.
What a negative or faint C line means on an antibody test:
- The test reagents are faulty
- The test was performed improperly
- The sample was too diluted or insufficient
- The sample was collected at the wrong time relative to onset of symptoms
If the C line is negative or faint, the antibody test cannot provide any result and is considered invalid, regardless of whether the Covid line appears positive or not. The person would need to get retested with a new antibody test kit.
Conclusion
In summary, a negative or faint C line on any type of Covid test (PCR, antigen, or antibody) means the test is invalid and the results cannot be used. This indicates a problem with the test reagents or procedure, not the sample itself. The person would need to collect a new sample and run a new test to get an accurate result.
When to Retest After an Invalid Result
If your Covid test is invalid, meaning the C line did not appear properly, you should test again as soon as possible. Here are some general guidelines for retesting after an invalid result:
PCR Tests:
- Retest in 24-48 hours if symptomatic
- Retest in 5-7 days if asymptomatic after a known exposure
Rapid Antigen Tests:
- Retest the same day or the following day if symptomatic
- Retest in 3 days if asymptomatic after a known exposure
Antibody Tests:
- Retest right away with a new test kit if within 1 week of symptom onset or suspected exposure
- Retest in 7-14 days if testing too early
- Retest in 1-2 months if testing well past exposure to check for longer-term antibodies
Be sure to follow the manufacturer’s instructions for your specific test when retesting after an invalid result. Continue testing until you get a clearly valid result.
Tips to Avoid Invalid Results
While invalid results can happen for various reasons, here are some tips to help avoid them when using at-home test kits:
- Carefully read and follow all instructions provided with the test.
- Check the expiration date and do not use expired tests.
- Store tests properly to avoid damage (e.g. do not expose to high heat or freezing temps).
- Collect the sample correctly (e.g. nasal swab technique for PCR tests).
- Time the test accurately and read results at the correct interval.
- Avoid eating, drinking, or exercising right before sample collection.
If you follow the instructions closely, you are more likely to get an accurate, valid result from an at-home Covid test.
Troubleshooting Invalid COVID Test Results
If you continue to get invalid Covid test results, here are some things to try:
For at-home tests:
- Try a test from a different manufacturing lot number
- Try a different brand of test
- Contact the manufacturer for troubleshooting tips
For PCR and antigen tests performed at a testing site:
- Make sure the sample was collected and handled properly
- Ensure proper PPE and procedures were followed
- Try a test at a different testing facility
- Consider alternative test types (e.g. PCR instead of antigen)
Persistent invalids may indicate a bad batch of tests, so try using tests from different lots or brands. For lab-based tests, proper procedures must be followed carefully to get accurate results.
Can You Spread COVID-19 With an Invalid Test Result?
If your Covid test result comes back invalid, it is not possible to definitively determine from that test whether you are positive or negative for Covid. An invalid result means the test simply did not work, rather than indicating the sample was negative.
However, it is possible to spread Covid-19 while receiving an invalid test result in the following scenarios:
- You are tested too early before viral levels are detectable
- You are asymptomatic but still contagious
- You test incorrectly due to user error
Even with an invalid result, you should continue taking precautions and isolate from others until you can get a clearly valid test result.
Key stats:
Here are some key statistics to summarize the risk of spreading Covid-19 with an invalid test result:
- Asymptomatic individuals account for an estimated 40-45% of Covid infections
- People can transmit Covid 2-3 days before symptom onset
- Rapid tests detect Covid in 80-90% of contagious individuals
- An estimated 2-5% of self-administered rapid tests are performed incorrectly
Based on this data, it is possible to spread Covid to others while testing invalid, especially if asymptomatic, testing too early, or improper technique is used.
Table Summary of COVID Test Result Interpretation
Here is a table summarizing how to interpret results from different types of COVID-19 tests:
| Test Type | Positive Result | Negative Result | Invalid Result |
|---|---|---|---|
| PCR | COVID detected | No COVID detected | Retest – Result invalid |
| Antigen | COVID likely | COVID unlikely | Retest – Result invalid |
| Antibody | Prior infection likely | No prior infection | Retest – Result invalid |
As shown, an invalid or faint C line on any COVID test means the result cannot be interpreted and retesting is necessary.
Frequently Asked Questions
Why did my COVID test come back invalid?
Common reasons for an invalid COVID test result include:
- Issues with the test reagents or chemicals
- Improper sample collection or handling
- Mistakes performing the test
- Testing too early or late in infection
- Manufacturing defects in the test materials
- Test stored incorrectly leading to damage
How accurate are COVID tests with faint lines?
COVID tests with faint or ambiguous lines are considered invalid regardless of the results. Even faint C lines indicate issues with the test reagents, while faint T lines make the results inconclusive. Invalid tests should not be relied upon for accuracy.
Should I isolate after an invalid COVID test?
Yes, you should continue isolating after an invalid COVID test result until you can be retested with a clearly valid result. Since an invalid result does not give a clear positive or negative, it is safest to act as if you could be contagious until ruled out with a valid test.
Can I get COVID if my test result is invalid?
Yes, it is possible to have COVID-19 even with an invalid test result. An invalid result simply means the test malfunctioned, not that you are necessarily negative for COVID. Continue isolating and retest as soon as possible after receiving an invalid result.
How long after an invalid COVID test should I retest?
You should retest for COVID immediately after receiving an invalid result, as long as a new test is available. Follow your doctor’s advice on the appropriate retesting interval based on your symptoms and risk factors.