It can be confusing to receive a positive test result only to find a negative one shortly after. Understanding the reasons behind such conflicting outcomes can clarify the situation and guide next steps. This article explores common factors contributing to a positive test followed by a negative, covering biological processes and test-related considerations.
Understanding Test Mechanisms
Diagnostic tests identify specific markers in the body, such as hormones or viral components. Each test detects these markers when their concentration reaches a detection threshold. If the marker’s concentration is below this threshold, the test may not register a positive result.
Tests have sensitivity and specificity. Sensitivity is a test’s ability to correctly identify individuals with the condition (true positives), minimizing false negatives. Specificity measures a test’s ability to correctly identify individuals without the condition (true negatives), reducing false positives. Different tests, even for the same condition, can have varying sensitivities, meaning some detect lower marker levels.
Biological Influences on Results
A common biological reason for a positive test turning negative, particularly in early pregnancy, is a “chemical pregnancy.” This occurs when a fertilized egg implants, producing human chorionic gonadotropin (hCG). However, the embryo stops developing very early, usually within the first five weeks. hCG levels rise enough for an initial positive test but then decline as the pregnancy is not viable, leading to a subsequent negative result. This is a very early miscarriage, often caused by chromosomal problems.
Similar fluctuations occur with other biological markers, such as viral loads during an infection. In early stages, the virus amount can increase rapidly, triggering a positive test. However, the immune system may quickly clear the infection, causing the viral load to drop below the detection threshold. For example, viral loads can be very high in the first week after diagnosis but decrease significantly by the third week. This rapid clearance can result in a positive test followed by a negative one, even if an infection was briefly present.
Other factors can also temporarily elevate marker levels. For instance, certain fertility medications containing hCG can lead to a temporary positive pregnancy test, which then turns negative as the medication leaves the system. The body might also produce a marker for a brief period due to a minor, self-resolving physiological event, causing a fleeting positive result before it disappears.
Test and User-Related Factors
Discrepancies in test results often stem from differences in the tests themselves or how they are used. Varying sensitivity between different test types or brands is a significant factor. For example, a highly sensitive laboratory test might detect a very low marker level, yielding a positive result, while a less sensitive at-home test taken later may not, resulting in a negative outcome.
User error during testing is another common cause for conflicting results. This includes not following instructions precisely, such as using an incorrect sample volume or reading the result outside the recommended time frame. Reading a test too early might show a faint line that later disappears, or reading it too late can lead to “evaporation lines” mistaken for a positive result. For tests like pregnancy tests, collecting a urine sample after consuming large fluids can dilute the marker, potentially leading to a false negative.
Issues with the test kits themselves can also contribute to inaccurate results. Expired or improperly stored tests may not function correctly, leading to unreliable outcomes. A faulty test kit could also produce an erroneous positive or negative result. In some cases, particularly with pregnancy tests, the “hook effect” can occur. This happens when extremely high concentrations of the detected marker, such as hCG, overwhelm the test’s ability to bind properly, paradoxically leading to a false negative or a very faint positive line. This effect is uncommon in early pregnancy but can occur with very high hCG levels.
What to Do Next
If you receive a positive test result followed by a negative one, retesting is often the first recommended step. Carefully follow all instructions, paying close attention to timing and sample collection. Using a different brand or type of test for reconfirmation, if available, can also provide a broader perspective.
If conflicting results persist, or if you have ongoing symptoms or concerns, consult a healthcare professional. They can offer further guidance, potentially recommending more sensitive laboratory tests or clinical evaluations to determine the underlying reason. Medical consultation is particularly important if you experience persistent symptoms, suspect a pregnancy complication, or have other health concerns.