Pregnancy is the attachment of the fertilized egg to the uterus wall and it is usually accompanied with hormonal changes to create the appropriate environment for the growth of a viable embryo.
A positive pregnancy test could be good news or bad news depending on the intention of the party involved. Also a positive or negative pregnancy test can affect patient care because not every drug can be given to pregnant women.
To avoid any form of psychological trauma associated with a pregnancy test, it is imperative to put adequate measures in place to get an accurate outcome/result of the test. Also, knowing various factors that can affect the outcome of the test would make one vigilant at interpreting the results when those factors are present or suspected to have an influence on the outcome of the test.
What is a pregnancy test?
A pregnancy test is a diagnostic test that is used to find out whether a woman is pregnant or not. Usually the test detects a hormone called human chorionic gonadotropin (hCG), the beta-subunit, which is produced by the placenta after the fertilized ovum (the zygote) is attached to the uterus wall.
The hCG stimulates the ovaries to produce progesterone to support the development of the embryo until the placenta is able to produce adequate amounts of progesterone. The hCG level rises and peaks at about 11 weeks of pregnancy. The hCG is detected both in the blood and in the urine.
Home pregnancy test usually involves the use of urine which is 97-99% accurate in detecting pregnancy as claimed by the manufacturers. The home pregnancy test is a qualitative assay which means it tells whether a woman is pregnant or not but does not show the level of the hormone or the age of the pregnancy.
The level of the hCG is detectable in urine at days 8-14 after implantation. The blood level of the hormone is usually higher as compared to the urine level. Therefore pregnancy can be detected earlier before the fourth completed week from the last menses when the serum is used. The blood test is done in health facilities where there are trained personnel.
The beta-subunit of the hormone is also produced in lower amounts in the testis, lung, liver, colon and stomach. The alpha-subunit of hCG is similar to thyroid stimulating hormone(TSH), luteinizing hormone(LH) and follicle-stimulating hormone (FSH). These hormones do not usually cross-react with the test device and therefore do not interfere with the test outcome.
How do I know whether the test is positive or negative?
The pregnancy test device usually comes as a card, cassette, or dipstick. It has a membrane which is impregnated with antibody against hCG at the test zone. There is also a control line usually depicted C on the card, cassette, dipstick. To perform the test, the cassette is placed on a flat surface.
Using a pipette, a clean urine sample (in a container obtained from a client seeking to test for the pregnancy) is pipetted and a drop or two are placed in the sample well of the test cassette and the results read within 10 minutes. The urine sample flows through the membrane by capillary action.
For a test to be declared positive or negative depends on the number of lines that appear on the cassette. The control line usually depicted C would usually show when enough urine flows through the membrane.
In a negative test, only the control line would show indicating the absence of the hCG hormone in the urine. For a positive test, the test line which is usually depicted with T would appear in addition to the control line C. This indicates that the hCG hormone is present in the urine indicating pregnancy.
Most test cassettes or cards detect hCG at 25IU/L and levels below it might not be detected by the test card or cassette which is false negative. the test is invalid when no line appears or when only the test line appears without the control. Such a test has to be repeated with a different test device.
For the dipstick, it is immersed vertically into the urine sample and held for 5 to 10 seconds before removing. The results can be read within 10 minutes.
The right way to read the Pregnancy Test Result
Can the test be wrong?
Yes. The test can be wrong. There can be a false positive test result and a false negative test result.
False negative test result is when one is pregnant and the test kit fails to detect the pregnancy. This would result when the level of hCG in urine is below the threshold at which the test kit is sensitive.
It has been established that the urine hCG levels in the early days of pregnancy usually ranges between 3-19950IU/L at the fourth completed week from the last menstrual period. So a test cassette or dipstick that detects pregnancy at 25IU/L level would definitely miss detecting a pregnancy where the hCG in urine is below that level (or threshold) that the cassette or dipstick detects.
Also, there is usually a diurnal variation in the level of hCG in urine and the highest level of the hormone is present usually in the morning. Though most test devices allow for carrying out the test at any moment, the preferred time is early in the morning when the hormone level is highest.
Factors that can lead to a false negative result
- Drinking a lot of water to dilute the urine can also produce false negative results.
- Not giving enough time for the reaction to occur or checking the results earlier can give false negative results.
- Another phenomenon is the ‘hook effect’ or ‘prozone effect’. The hook effect is when there are high amounts of the hormone present in the urine or blood and which overwhelms the test such that no hormone binds to or no hormone directly binds to the test reagent (antibody against the hCG) therefore producing false negative results. The ‘hook effect’ is a rare phenomenon.
- Another reason is when too much of the degraded product of the hormone, usually referred to as the hCG core fragments, is present in the urine (βcf-hCG). When the core fragment binds to the antibody, no reaction occurs and the visible line would not be shown.
For false positive results, the test indicates a pregnancy but in reality there is no pregnancy. A false positive result could be due to;
- Underlying medical problems of various body organs which results in the production of high amounts of the hCG hormone.
Medical problems can give a positive urine pregnancy test include germ cell ovarian tumors, gestational trophoblastic tumors, placental site trophoblastic tumours, ectopic pregnancy, paraneoplastic syndromes, urothelial bladder carcinoma, lung carcinoma, urogenital anatomical malformations, adenomyosis, metastatic melanoma, nephrotic syndrome following blood transfusion from a pregnant woman, when one is in the perimenopausal and postmenopausal states (the hCG is thought to be produced from the pituitary gland which may be a sign of a malignancy).
- Some medicines like the selective serotonin reuptake inhibitors (SSRI). Specifically escitalopram.
Which time of the day is best for the pregnancy test
The best time to detect an early pregnancy is early in the morning. The early morning sample contains the highest amount of the hCG.
However, as the pregnancy progresses the levels of the hormone rises very quickly such that high amounts of the hormone is present at any moment of the day and therefore permits testing at any moment of the day.
In case one fails to perform an early pregnancy test in the morning, you may perform the test in course of the day by not peeing for at least four hours prior to the test or avoid drinking too much water or fluids which would dilute the urine and therefore diluting the pregnancy hormone making it impossible to detect the hormone in the urine.
How soon after an intercourse can I test for pregnancy
Early pregnancy test is usually done on the first day or first week when one misses her menstrual period. This includes the fourth completed week since the last menstrual period to the sixth day of the first missed menstrual period (that is 4weeks 0 days to 4 weeks 6 days).
Testing for early pregnancy at this time would yield the most accurate results. Also, one can screen for pregnancy at least ten days after intercourse if the person does not want to wait till she misses her period. However, this might lead to false negative results and should be repeated at 2-7 days.
The sperm can live up to five days after unprotected vaginal sexual intercourse. This means fertilization would not necessarily occur the day after intercourse. After fertilization it takes several days for implantation to occur (5-18 days, averagely 14 days).
After implantation it takes about 7 for the hCG to be detectable in urine. This indicates that to obtain accurate results for detecting early pregnancy with the home pregnancy test device one should test in the week of the missed menstrual period.
Should I repeat the pregnancy test in case it is negative?
Yes. It is advisable to repeat the test 2-7 days after a negative pregnancy test when one had an unprotected vaginal sex.
It could be that the test was done too early and the hCG was below the level the test device is sensitive to, the test result was read too early, a diluted urine was used (early morning urine is preferred), the device is substandard or expired, hook effect/prozone effect that is observed (diluted urine would be preferred).
One can also go in for the serum hCG test which is usually done by a qualified health professional.
Can the test be positive even when I am not pregnant?
There are instances where false positive results have been obtained. The following are some causes of a false positive pregnancy test.
2. Ectopic pregnancy
3. Evaporation lines (these are usually colorless or almost colorless lines which are obtained when the test is allowed to sit for too long before reading is taken.)
4. Some medications such as
The above medications are administered as “trigger shot” to mature ova and trigger their release following a course of clomiphene. They are synthetic hormones of the hCG and take about 10 days to be cleared from the body. Another synthetic hormone known to cause a false positive pregnancy test is the human menopausal gonadotropin (hMG). It is a combination of luteinizing hormone (LH) and follicle stimulating hormone (FSH). The surge in the LH can cause a false positive pregnancy test due to its structural similarity with the hCG. These medications are used in in-vitro fertilization or intrauterine insemination (IUI).
Other medications are
Some particular medical conditions which the patient might be suffering from can give a positive urine pregnancy test.
· Germ cell ovarian tumors
· Gestational trophoblastic tumors
· Placental site trophoblastic tumors
· Paraneoplastic syndromes
· Urothelial bladder carcinoma
· Tubo-ovarian abscess
· Lung carcinoma
· Colon cancer
· Metastatic melanoma
· Breast cancer
The above trophoblastic and non-trophoblastic neoplasms can cause elevation of hCG and its excretion in urine.
· Ectopic pregnancy
· Urogenital anatomical malformations
· Urinary tract infection (obstructive pyelonephritis)
· Nephrotic syndrome-an inflammatory disease that affects the glomeruli of the nephrons that cause a lot of proteins to leak into urine. It is characterized by oedema, hyperlipidemia, and hypertension. Proteinuria of 4+ can give a false positive pregnancy test and is usually caused by rheumatoid factor in the urine. To screen for pregnancy in such a situation it is advisable to use the serum.
Another instance of a false positive pregnancy test is when the container that is used to collect the urine of a pregnant woman is not rinsed but used to pick the urine sample of a non-pregnant woman to run a pregnancy test.
Butler A.Stephen , Sara A. Khanlian And Lawrence A.Cole (2002), Sensitivity of Home Pregnancy Test devices, clinical chemistry 47, No 12.
Sarah Johnson, Saji Eapen, Peter Smith, Graham Warren and Michael Zinaman (2017), Significance of pregnancy test false negative results due to elevated levels of beta-core fragment hCG, journal of immunoassay and immunochemistry, 34:4, 449-455,
Rahul Mital, Moriah Forster, Abdurahman Alloghbi and Ammar Kayyali (n.d), A case of a false-positive urine pregnancy test and delayed diagnosis of obstructive pyelonephritis, department of internal medicine, University of Toledo, Toledo, OH, USAs