Published: 3 September 2023
Pregnancy or “hCG” testing is one of the most common tests performed on women, especially those of typical child-bearing age. Listed below are some answers to common questions related to basic pregnancy testing principles.
What is Human Chorionic Gonadotropin (hCG)?
This is a hormone that is secreted in order to develop a woman’s womb during pregnancy and ensure there is an ideal environment for carrying a baby.
How soon after unprotected sex can I test for pregnancy?
The day after you’ve missed your period or at least 2-3 weeks from intercourse is a good estimate for when hCG levels should be high enough to trigger a positive result. Contrary to popular belief, pregnancy does not occur immediately after sex, rather it is a series of events that takes weeks to carry out. This is because the sperm will first need about 5-6 days to fertilize the egg, after which it will take another 3-4 days to move from the fallopian tube to the uterus and once there, another 3-4 days to implant itself into the uterine lining. Once the egg is implanted, it may take another 2-3 days for the hCG levels to reach high enough for a positive result. This time will vary depending on what type of test you are using since some tests are more sensitive than others.
How long after implantation does hCG rise?
hCG begins rising immediately upon implantation in the uterus and will continue to double every 2-3 days until it peaks at the end of the first trimester between 8-12 weeks. After this point, hCG levels in the blood will begin to gradually decrease until the time of delivery. After delivery, levels will drop off rapidly until they are at “undetectable levels”.
There are general ranges for hCG levels which correspond to varying gestational periods, but there are instances where these levels may be higher than with a typical pregnancy. One common reason is when multiple fetuses are present in the placenta, such as with twins, triplets, etc. Another could be due to the presence of certain hCG-producing tumors like choriocarcinomas. Even men can have levels high enough to trigger a positive pregnancy test in events like testicular cancer.
How do home pregnancy tests work?
Pregnancy tests which are taken at home fall under the category of qualitative testing. They are also known as Enzyme-Linked Immunosorbent Assays (ELISA) and they utilize a sandwich technique to visualize positive results. This technique involves a lateral flow strip being impregnated with enzyme-linked antibodies. When a urine or blood sample moves along the strip, any hCG that is present will bind to those antibodies and form an antigen-antibody complex that results in a colored line and a positive result.
Qualitative vs. Quantitative hCG testing
A qualitative test gives a simple “Yes or No” answer as to if you are pregnant, whereas a quantitative test tells you the actual amount of hCG in your body. Despite this extra information, it shouldn’t be used to date how far along a pregnancy is, since the range can vary so widely. It is not the actual number that should be paid attention to, rather the trajectory, since the up or down movement can be predictive of different outcomes.
Qualitative Test
A qualitative test is one that you are likely to perform yourself at home with urine and is usually interpreted by seeing a colored line within 1 to 5 minutes of adding the specimen. You can also use blood with a qualitative test, but you will require a doctor visit to get your blood drawn and spun down in a centrifuge. Most of these tests are capable of detection at about 25 mIU/ml or above.
Quantitative Test
- A quantitative test tells you the exact amount of hCG in “milli-international units per milliliter” (mIU/ml), thus it must be run on a chemistry analyzer in a laboratory and is not something you will run at home. The specimen of choice is serum or plasma, so you will need to get your blood drawn for this type of test. The run time on these are longer than with qualitative tests, usually taking about 20-30 minutes to run. Any shipping or processing will add time to the process as well. With that considered, it could mean you won’t know your result for 1-3 days, possibly longer. Quantitative analyzers can detect hCG down to 1 mIU/ml or below, although many facilities consider anything below 5 mIU/ml as a negative result for pregnancy.
Quality Control in HCG testing
It is imperative to perform Quality Control (QC) to ensure the accuracy of your results during testing. There are two main forms of QC to consider: internal and external:
Internal QC
Most people are most familiar with Internal QC since this is the “Control” line you see when performing qualitative testing with at home tests and is the reason that most positive tests will show two lines (the first line is the control line and the second line is the positive test line). In the event of a negative test, it would still show one line, which would be the control line. If there are no lines at all, the test should be deemed invalid and repeated with a new test. This is the manufacturers’ way of letting you know that something went wrong and the results shouldn’t be used. Internal QC is performed automatically once the specimen is added and doesn’t usually require intervention by the tester. There are many reasons that internal QC can fail, though many cases point back to the actual test device or the sample.
External QC
External QC uses an “external” substance to verify the integrity of a test and is performed on both qualitative and quantitative methods.
Qualitative Testing
For qualitative testing, these external controls will typically consist of a vial containing hCG for the positive control and a vial without it as the negative control. An aliquot of each control is added to separate test devices and each one should result appropriately as positive and negative. If either one does not result as expected, troubleshooting will be needed.
Quantitative Testing
For quantitative testing, there will normally be vials designated as low, middle and high levels with each level having an expected range of concentration. If each result falls within its range, then QC passes, if not, troubleshooting will be necessary.
Are there special dietary restrictions to follow before taking a pregnancy test?
No, you will not need to be fasting or perform any preparation prior to taking the test. However, its a good idea not to overhydrate with liquids prior to urinating to avoid diluting your specimen and causing potential false negative results. Both blood and urine specimens can be used, but blood is preferred over urine since a positive can be detected earlier due to higher concentrations of hCG. Unlike with urine tests, for blood tests, you will have to see a doctor since you will need to get your blood drawn. Blood specimens should be drawn in either gold, red or green top tubes.
What causes a false negative pregnancy test?
When using urine as the specimen of choice, a false negative result can occur for a few reasons.
If the specific gravity (amount of dissolved solids) is too low. Using the first morning void can help prevent this since the specimen will be more concentrated.
Testing too early in the pregnancy can cause false negatives even with blood samples since the hCG levels have not been increasing long enough to trigger the threshold of the test.
A low hCG may also be an early clue to possible complications like an ectopic pregnancy or miscarriage.
What causes a false positive test?
There are times when a pregnancy test will read false positive even though the woman is not pregnant.
- This could be linked to certain medications the mother is taking, especially fertility related ones containing HCG.
- Another reason can be when you’ve waited too long to read the test instead of reading it at the recommended time in accordance with the instructions. This is due to very faint grey to colorless lines called “evaporation lines” which look like a very weak positive result. Do evaporation lines disappear? No, they do not. The best way to avoid this problem is by reading the test at the prescribed time in accordance with the instructions. Also, using tests that have pink dye are considered superior since blue dye tests have higher incidence of resulting in evaporation lines.
- Another phenomenon which can cause false positive results is something called “phantom HCG”. This is when people have rare antibodies which react with the materials of the test and give a positive result with blood, mimicking the action of hCG. However, despite the blood test being positive, running a urine test will instead show a negative result, since the antibodies are large protein molecules that are too big to be excreted by the kidneys into urine.
HCG levels for multiples?
hCG levels can give clues of whether an expectant mother is carrying either a single fetus or multiple. In cases where there are twins, triplets or even more, these levels will often be significantly higher than for a mother carrying a single fetus. However, a single HCG test on its own should never be the single deciding factor for this. This should be accompanied by ultrasound under the supervision of a physician.
When is the best time to take a pregnancy test, morning or night?
The best time to take a pregnancy test is in the morning right after waking up. This is because the hCG levels should be more concentrated due to lack of hydrating and urinating during the sleep cycle. This can make a big difference if you are early on and already barely meeting the minimum cutoff for your testing kit. Keep in mind, this will vary from person to person. Example, for someone working night shifts, their morning might be 5 p.m. since their sleep cycle was during the daytime instead of the night.
Conclusion
hCG is a common test which can be done in the doctor’s office or at home. Whether it is a qualitative or quantitative test will depend on your reason for needing the test. Remember that qualitative testing is quicker and can be done conveniently at home. Whereas, quantitative testing in a doctor office will be more sensitive, gives a more detailed result and can help monitor the progress of a pregnancy.
References
Leeuwen, V., & Bladh. (2019). Davis’s Comprehensive Manual of Laboratory & Diagnostic Tests with Nursing Implications. In V. Leeuwen, & Bladh, Davis’s Comprehensive Manual of Laboratory & Diagnostic Tests with Nursing Implications (p. 373).
