Last Updated on October 1, 2021
The home pregnancy test kit is a commercially available kit that women can use to test the signs suggestive of pregnancy.
A home pregnancy test kit measures the amount of human chorionic gonadotropin (hCG) present in the urine of the pregnant woman.
There are many types of commercially available kits under different brand names but all work on a similar principle.
What is HCG?
HCG is a hormone produced by the cells of the placenta. The placenta is a temporary organ that develops in pregnant patients and connects the developing fetus to the wall of the uterus. The fetus takes nutrition from the mother’s body as well as expels waste products into the mother’s blood circulation through the placenta.
Placenta also produces hormones that support the pregnancy. These include hCG (human chorionic gonadotropin), estrogen, progesterone, and human placental lactogen.
HCG provides nourishment to the egg after it has been fertilized and attached to the uterine wall. Its main function is to sustain and maintain a pregnancy after conception.
[Read more about HCG]
Principle Behind Home Pregnancy Tests
HCG can first be detected in a blood sample about 11 days after conception or 3 weeks after the first day of the last missed period. Levels continue to double every 48 to 72 hours. They reach their peak at around 8 to 12 weeks of pregnancy. After that, the values start declining and level off, remaining steady for the rest of the pregnancy.
Since hCG levels start rising immediately after successful conception, its level in blood and urine can be used to diagnose and confirm pregnancy.
Women can test positive for pregnancy owing to hCG production several days before a missed period.
The most accurate method of detection of hCG is through a blood test which gives a quantitative measurement of hCG levels.
HCG levels can also be estimated in the urine of pregnant patients. This method gives a qualitative estimate of hCG.
Although less sensitive than blood testing, a qualitative estimate of hCG level in urine can give a fair indication of whether the woman is pregnant or not.
Home pregnancy urine dipstick tests are based on this principle.
It employs chromatographic immunoassay and is based on the lateral flow technique. It is a qualitative test.
The detection thresholds range from 20 to 100 mIU/ml, depending on the brand of test. In early pregnancy, when the levels of hCG are low, a positive result may be obtained by using the first urine of the morning (which is most concentrated).
It is very convenient, can be performed within the confines of home, and ensures complete privacy.
How Accurate are Home Pregnancy Test Kits?
If carried out according to the manufacturer’s instructions provided along with the kit, pregnancy detection kits are highly accurate (99% accuracy).
Various brands have different detection thresholds and some kits are not sensitive enough to detect low hCG levels in early pregnancy (before or on the day of missed period). When carried out few days after the missed period, almost all the test kits provide a satisfactory test result.
What is Ideal Time to do a Home Pregnancy Test
Home pregnancy test kits are designed to detect levels of hCG produced in case of pregnancy. Since the body starts producing hCG approximately 3 weeks after the last menstrual period, levels of hCG become detectable before the next due period.
However, the ideal time to test is on the first day of the missed period or a few days thereafter.
If performed earlier than the first day of the missed period, there are more chances of an inaccurate result.
The result may be false negative as the levels of hCG are not high enough in early pregnancy. Conversely, in some cases, the test may show a positive result but pregnancy may not develop successfully, due to spontaneous miscarriage.
Ideal Sample for Pregnancy Test
The level of hCG is highest in the early morning because urine is most concentrated at this time. So in the early days of pregnancy, there are higher chances of an accurate result, if the test is carried out using the first-morning urine sample. Similarly, excessive fluid intake before taking the pregnancy test can dilute (hence lower) the hCG level in urine and the level may not be detected resulting in a false-negative result.
Affect of Medication on Pregnancy Test Results
Intake of antibiotics, painkillers, or birth control pills does not affect the test result.
If a patient is receiving any medication which contains hCG, the test result will be falsely positive. In addition, few drugs like methadone, chlordiazepoxide, or promethazine can also give a false positive pregnancy test result.
How to Purchase and Store a Home Pregnancy Test Kit
Most of the kits work on the same principle. So brand becomes a personal choice. Some brands are less sensitive in the early days than others. Please go through the user leaflet to find that.
Otherwise, any kit can be purchased as they all work on the same principle of detecting hCG level.
The important thing is to check the expiry date on the box and to ensure that the box is fully intact, with no wear and tear.
Once bought, the kit needs to be kept in the refrigerator [not in freezer comparment] till the time of testing.
The kit should be taken out approximately half an hour before performing the test so as to bring it to room temperature.
Home Pregnancy Test Procedure
Before you do the test, please read the instructions carefully. It is very important to read and follow the instructions manual provided along with the kit. Although most pregnancy urine tests are the same, there may be minor differences like the method of collection of a urine sample and the time after which the result has to be read.
- Wash your hands with soap and water
- Open the test kit pouch and take out all the contents of the pouch. Avoid touching the absorbent pad of the test kit.
- Collect urine in a clean [preferebaly sterile, available commercially or provided with kit] container.
- Discard the initial few ml of urine to collect a midstream sample of urine. This ensures that the urine collected is sterile and not contaminated by normal micro-organisms present on the external genitalia.
- A dropper is provided in the kit. Use this dropper to place a few drops of urine into the well of the test stick [refree to manufacturer’s instructions].
- Immediately, the urine will start spreading on the blotting paper and move towards the direction of the result window.
- The testing stick is then placed on a clean, level surface with the result window facing up. Wait time is usually between 1 to 5 minutes [see instructions]
Some kits have an absorbent end that is dipped in urine instead of being dropped by a dropper. Still, others may require the woman to urinate directly on the stick. In such cases, the urine stick should be held for a specific time period. It is important to place the absorbent end of the stick towards the urine stream.
Interpretation of Results of Home Pregnancy Test
After the time mentioned in the instructions has passed, the test stick should be checked for results.
Pregnancy is indicated by a change in color in the designated area of the device. The display of results varies in different types of pregnancy test kits.
In most kits, it is displayed by color bands (lines).
The presence of two colored lines corresponding to C (for control) and (T for test) indicates a positive result.
The presence of only one colored line corresponding to the C position indicates a negative result.
The absence of a colored line at both C and T or the presence of the colored line at only T position is interpreted as an invalid test. This indicates a faulty test kit and the test should be repeated with a fresh kit.
In some pregnancy test kits, the result is displayed in the form of plus (+) and minus (-) signs.
A faint colored line at the T position indicates a positive result. It could be due to a low level of hCG which is usually present in early pregnancy. Test carried out after a few days will usually show clear positivity as the hCG level will rise in urine.
If the results are positive
The woman should consult the doctor to have the pregnancy confirmed. This can be done by pelvic examination, blood tests, and ultrasonography.
If the results are negative
The woman should wait for 2 to 3 days. If menses have still not started, the test should be repeated again. False negatives are fairly common, especially if the ovulation date has been miscalculated or the test was carried out too soon. If the second test is also negative, the doctor should be consulted to find out if there is some other problem affecting menstruation or causing symptoms of pregnancy.
False-negative Pregnancy Results
A negative result usually indicates that the patient is not pregnant. However if performed in early pregnancy, the hCG level may be quite low and not detectable by conventional tests. In such cases, it is advised to repeat the test after 48-72 hours.
False-positive Pregnancy Results
Exogenous hCG administration:
It results in a high hCG level for up to two to three weeks after the drug has been administered.
Improper Testing and Interpretation:
It is important to follow the instructions properly as mentioned in the diagnostic kit. Also, the test result should be interpreted within the time period mentioned in the kit. Some tests produce a faint false-positive test result if read after the instructed time.
Defective Pregnancy Test kit :
Expired tests or faulty home pregnancy tests can also lead to false-positive results.
This is a pregnancy that gives a positive pregnancy test but the embryo or fetus does not develop.
Very early pregnancy loss or miscarriage can also result in a false-positive test result.
Certain Medical Conditions:
HCG levels are also increased in gestational trophoblastic diseases as well as in tumors of the ovary and testes. Besides, certain other conditions like pancreas islet-cell tumors, adenomyosis, gonadoblastoma, bladder cancer, lung cancer, etc may lead to elevated hCG levels. All these conditions will produce false-positive pregnancy test results.
Some drugs like methadone, chlordiazepoxide, or promethazine can give a false positive pregnancy test.
A rare psychological condition in which the patient falsely believes that she is pregnant. It results in altered hormone levels in the blood resulting in a false-positive test result.