Pregnancy tests work by detecting the presence of human chorionic gonadotropin (hCG) hormone in a woman’s urine. This hormone is produced by the placenta shortly after implantation occurs, which is about 6-12 days after conception. Pregnancy tests detect even very low levels of hCG, with current tests able to detect hCG levels as low as 25 mIU/mL.
The sensitivity of pregnancy tests has improved greatly over the years. Early pregnancy tests required hCG levels of up to 1,000 mIU/mL to turn positive, while current tests can detect much lower levels. This advancement allows pregnancy to be detected earlier.
So will a pregnancy test still turn positive if left to sit for awhile after it has been taken? The answer depends on several factors.
Does hCG Level Degrade Over Time?
Human chorionic gonadotropin is a protein-based hormone. Like other proteins, hCG can degrade over time, especially when exposed to heat, light, air, and bacterial contamination. The level of hCG measured in a urine sample will gradually decline if left sitting at room temperature or warmer.
However, hCG degrades slowly. One study found that hCG level declined by only 8.1% over 24 hours when urine samples were left at room temperature.1 Refrigerating the sample helped preserve hCG level.
So while hCG level does drop when urine is left sitting, the decline is generally slow under normal conditions. The level does not normally drop enough to change a positive pregnancy test to negative in just a day or two.
Test Sensitivity and hCG Threshold
Whether a test turns positive or negative is determined by the sensitivity of the test. Most home pregnancy tests today can detect hCG levels of 25 mIU/mL and above.
This means that as long as the hCG level remains above 25 mIU/mL, the test line should still appear positive regardless of sitting time. The level has to drop below the threshold for the test line to disappear.
For a pregnancy test to turn from positive to negative, the hCG level would have to significantly degrade to less than 25 mIU/mL. For most cases, it is unlikely hCG would drop that much within a couple days at room temperature.
However, if the sample was left for very long periods (weeks) or at high temperatures, the hCG could potentially degrade below the test threshold.
Evaporation and Sample Concentration
When urine is left sitting in a cup, the liquid evaporates over time. This concentrates any hCG in the remaining sample.
Concentrating the sample slightly raises the hCG concentration. This makes the test more likely to remain positive.
So evaporation counters some of the impact of hCG degradation.
Dilution of Urine
If the urine sample became diluted after initial testing, such as adding water, the hCG concentration would drop. Diluting the sample could potentially lower hCG below the test threshold and cause a positive to turn negative.
However, small amounts of dilution are unlikely to change the result within a day or two. Significant dilution would be required to impact the test outcome.
Test Brand and Design
Not all home pregnancy tests are designed the same. Some brands may be more sensitive, with lower hCG thresholds, compared to other brands. Some styles also use different test strip designs and formats.
These differences could potentially impact how long a test remains positive after sitting. However, for most quality branded tests, the differences are likely insignificant in most cases.
Interpreting Faint Lines
Many pregnancy tests show two lines when positive – a control line and a test line. The test line may start faint in early pregnancy when hCG is still low. It then darkens as hCG rises.
A faint test line may persist or remain visible on a test that has been sitting out for awhile. But it may look lighter than the original result.
It’s important to read the test when initially completed and note if a faint line was present. Don’t rely on interpretation of faint lines after the test has sat, as the line intensity can fade.
Impact of Sitting Time on Different Pregnancy Test Types
There are five main types of pregnancy tests: cassette/strip style, midstream, digital, early detection and late period tests. Does sitting time affect each test type differently?
Cassette and Strip Tests
These simple paper strip tests, where you either collect urine in a cup or dip the strip briefly, are most impacted by sitting time. The actual hCG binding strip is exposed to air and more prone to contamination compared to other enclosed test formats. Strip tests are also more prone to evaporation and interpreting faint lines.
Midstream Tests
Midstream pregnancy tests allow you to hold the absorbent tip right in the urine stream. Enclosing the absorbent tip may help protect hCG binding better than strip tests. But once sample is absorbed, the test strip is still exposed to air just like standard tests. Results could still be impacted by sitting time.
Digital Tests
Digital pregnancy tests encase the absorbent strip inside a plastic holder. This protects the strip and prevents evaporation or disturbing the strip once sample is absorbed. The digital readout is also less subjective than interpreting lines. For these reasons, digital test results may be less impacted by sitting time compared to non-digital versions.
Early Detection Tests
Early detection pregnancy tests use special strips that are extremely sensitive, typically able to detect hCG around 10-15 mIU/mL. The test strip itself is no different than other strip tests. So sitting time impacts early detection tests similarly to other strip tests in most cases. The only exception would be if hCG level was very close to the threshold initially.
Late Period Tests
So called “late period” tests are marketed to women after a missed period rather than for early testing. However, there is no real technological difference compared to standard tests. Late period test results are impacted by sitting time the same as other strip and midstream tests.
How Long Before a Positive Turns Negative?
Given hCG’s slow degradation and high sensitivity of current tests, it is unlikely a positive pregnancy test would turn negative in just 1-2 days.
However, there are some exceptions where a positive may turn negative quicker:
– The urine sample was extremely diluted after initial testing
– The urine sat at hot temperatures for an extended time
– The initial hCG level was extremely low (close to the 25 mIU/mL threshold)
– Evaporation was excessive, concentrating the sample too much
– The test strip was disturbed or contaminated after initial testing
Under typical conditions, it would likely take at least several days, possibly up to a week before hCG degraded enough for a positive pregnancy test to turn negative when left sitting out.
Proper storage is important if needing to re-check or confirm results at a later date. The sample should be refrigerated and sealed tightly in a clean container to preserve hCG level.
Tips to Get Reliable Results
To get the most accurate pregnancy test results that won’t be impacted by sitting:
– Check the test at the recommended time once sample is applied, without significant delay.
– Note if a faint line is present for re-testing later.
– Do not let the urine sample sit out at room or higher temperatures for more than a day before retesting.
– Refrigerate and seal sample tightly if needing to recheck more than 48 hours later.
– Do not dilute, contaminate or disturb the sample after initial testing.
– Do not make judgments on line intensity changes after sample has sat.
– Consider using a digital test if needing to confirm a positive after a delay.
Conclusion
In summary, while hCG level can degrade over time, pregnancy tests are sensitive enough that results typically remain positive for 1-2 days. But improper storage and delaying interpretation can impact results. To get the most accurate outcome, the test must be read at the proper time without long delays. Retesting after sitting should be done cautiously, understanding that faint lines or changes in line intensity may not be reliable. Properly storing the sample can help maintain hCG level for a longer period if rechecking the results is needed.