Urine Pregnancy Test: Introduction, Principle, Procedure, Result Interpretat

Urine Pregnancy Test: Introduction, Principle, Procedure, Result Interpretation and Keynotes

Introduction of Urine Pregnancy Test

Pregnancy can be confirmed in the laboratory by taking urine samples as well as blood (serum). In both specimens, there is the presence of a hormone, human chorionic gonadotropin (hCG). It is a glycoprotein hormone. The Urine pregnancy test (UPT) is simple, rapid, and easy to perform even in your home. HCG appears in the urine of pregnant women within 2 weeks, after the first missed menstrual cycle. UPT works on the principle of immunochromatography that points to qualitative detection of hCG in urine for the early detection of pregnancy.

Clinical Significance of HCG (human chorionic gonadotropin)

Human chorionic gonadotropin (hCG) is a hormone also called beta subunit of human chorionic gonadotropin (β-hCG) released immediately after fertilization by the developing placenta. hCG can be discovered in urine as early as 7 to 10 days after fertilization in a normal pregnancy. hCG levels grow quickly, frequently topping 100mIU/mL before the first missing menstrual period and peaking at 100,000-200,000 mIU/mL at 10-12 weeks. Because of its emergence soon after conception and subsequent rise in concentration throughout early gestational growth, hCG is a good early pregnancy marker

Principle of Urine Pregnancy Test

It works on the principle of immunochromatography. The immunochromatographic test principle is the same as the sandwich ELISA method. The only difference is that immunological reaction is carried out on the chromatographic paper by capillary action. For this system, two kinds of specific antibodies against the antigen are used. One of the antibodies is immobilized on the chromatographic paper while another is labeled with colloidal gold and infiltrated into a sample pad. An immunochromatographic unit is completed by attaching the sample pad at the end of the membrane. The liquid sample is dropped on the sample pad. The antigen in the sample forms an immunocomplex with the antibody labeled with colloidal gold. Its complex moves along with the liquid sample and makes a contact with the antibody immobilized on the membrane. It is followed by forming an immuno- complex with the immobilized antibody. It results in generating a colored red-purple line. The appearance of a red-purple line on the membrane indicates the presence of antigen of interest in the sample. The urine migrates through the membrane very fast, it makes it possible to detect the presence or absence of antigen (hCG) within 15 minutes as shown above image.

Components of Immunochromatographic test device

Sample pad: It acts as a sponge and holds an excess of sample fluid. Once soaked, the fluid migrates to the second element.
Conjugate pad: A dried format of bio-active particles in a salt-sugar matrix that contains everything to guarantee an optimized chemical reaction between the target molecule (e.g. an antigen) and its chemical partner (e.g., antibody) that has been immobilized on the particle’s surface.

Control: It contains antibodies (goat polyclonal antibodies) that pick up free latex/ colloidal gold in order to confirm the test has operated correctly.

Test: It contains a specific capture molecule and only captures those particles onto which an analyte (antigen or antibody ) molecule has been immobilized.

Specimen for Urine Pregnancy Test

Any random urine sample is collected in a clean and dry container that should be free from detergent. It must be collected at least 12 days after the first missed menstrual cycle. In the case of negative results repeat the test after one week.

Requirements for Urine Pregnancy Test

A commercial test kit that contains a test device and dropper.
Urine sample
Timer
Gloves
Waste disposing of bin

Procedure of Urine Pregnancy Test

Test kits are storable at room temperature.
Remove the test device from the pouch and use it as soon as possible.
Put the test device on a clean and level surface.
Hold the dropper vertically and transfer 3 full drops of urine to the specimen well and start the timer. Note- Avoid air bubble formation.
Wait for the band/s to appear.
Read the result within 15 minutes.

Result Interpretation of Urine Pregnancy Test

Test negative: Only on the band at the control region (C)
Test positive: Both bands at test (T) and control regions (C)
Test invalid: No band at all or band only at the test region

Keynotes on UPT

In a very high level of hCG (>500,000 mlU/ml), a false negative result may occur due to a “Prozone” effect. If pregnancy is doubtful, simply dilute specimen 1:1 with deionized water and repeat the test.
Urine with low specific gravity (diluted) may not contain a representative level of hCG. If pregnancy is doubtful, a first-morning urine sample should be obtained and retested 48 hours later.
A false positive UPT may occur in conditions like trophoblastic disease and certain non-trophoblastic neoplasms including testicular tumors, prostate cancer, breast cancer, and lung cancer cause elevated levels of hCG.
Medication of drugs like chlorpromazine, promethazine, phenothiazines, methadone, aspirin, and carbamazepine may cause false-positive urine pregnancy tests.
Drugs containing hCG may interfere with the test and produce misleading results.
The detection threshold of UPT is 20 to 50 mIU/mL while the blood pregnancy test is 5 to 10 mIU/mL.
Obstetric ultrasonography is another tool that may also be used to detect and diagnose pregnancy.
A false-negative result may occur due to taking the test before there’s enough hCG to produce a positive result and miscalculating the timing of missed period.
The test’s expiration date is also mandatory to know prior to testing to get accurate results of the pregnancy test.
A blood pregnancy test is more sensitive than a urine pregnancy test.
MonolabTest kit is applicable to detect hCG in serum, plasma, and urine to conform to the pregnancy status.

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