Human chorionic gonadotropin (HCG) is a hormone secreted by placenta during pregnancy. Its production stimulates secretion of progesterone by the ovary. Adequate levels of progesterone are necessary for successful implantation and prevent any further release of egg from the ovary.
Human chorionic gonadotrophin is a glycoprotein, has alpha and beta sub units. The alpha subunit usually cross react with the alpha subunit of leutenizing hormone, however, the beta subunit is specific for HCG. It appears in urine, blood and amniotic fluid. The serum and urine level rise rapidly during gestation, reaching a peak at six to eight weeks, after which there is a steady decline.
Laboratory tests for pregnancy are based on the detection of human chorionic gonadotrophin hormone in serum or urine; mainly there are two types of test.
I. Biologic animal Bioassay (A-Z test)
This test is performed in laboratory animal (female mouse). I.e. Patient’s urine is injected in to a female mouse after certain period, the mouse will be killed and the ovary will be examined for sign of pregnancy. However, this test cannot be used for early diagnosis. Moreover it is time consuming and requires steady supply of laboratory animals.
II. Immunologic test
Immunologic test could be qualitative and quantitative. Qualitative estimation of HCG in urine is used for early detection and confirmation of pregnancy. Quantitative estimation of HCG in serum has of value in case of preeclamptic toxemia, hydatidiform mole and choriocarcinoma. Compared to biologic animal assay, immunologic test is less expensive and quicker test.
An early morning urine specimen is preferable because this is the most concentrated and contains the highest level of HCG. However, specimen collected at any time may be used with a specific gravity at least 1.010. Urine must be collected in a clean detergent free container. If it cannot be tested immediately, it should be refrigerated at 40°C for not longer than 48 hours. Specimen preserved with boric acid is also suitable for testing.
When tested, the urine and test reagents should be at room temperature. If the urine is cloudy it should be filtered or centrifuged and the supernatant fluid used. Specimens that are heavily contaminated or contains large amount of proteins or blood, are not usually suitable for testing.
Factors Affecting Pregnancy Tests
False Negative may occur in conditions like:
• Error in reading – inappropriate interpretation of procedure
• Test is performed too early-The concentration of HCG is below the sensitivity of the test, which is capable of detecting reliably. The sensitivity of a test, the recommended time of testing will be included in the information supplied by the manufacture.
• Urine too diluted -falsely low levels of HCG may be due to a diluted urine (low specific gravity)
• Ectopic pregnancy implantation of the ovum outside the uterine cavity
False positive may occur in conditions like
• Error in reading- inappropriate interpretation of test procedure
• Luteinizing hormone cross-reaction
• Test performed at time of ovulation or in menopausal women
• Proteinuria and hematuria
• Recent pregnancy -test performed less than 10 days after abortion of full-term delivery.
• Detergents on glassware and slide used in the test, it must be well rinsed to remove trace of detergent even the smallest trace of detergents may affect the performance of the test.
• Drug interference- aldomet, marijuana, aspirin in large doses, etc.
• HCG treatment for infertility
• Trophoblastic disease e.g. molar pregnancy or choriocarcinoma
• HCG secreted by malignant tumor (ovary, breast, lung, kidney)
• Testicular tumor (in male)
Situations in which pregnancy testing is indicated include pregnancy test usually ordered to investigate some conditions like ectopic pregnancy, threatened abortion, hydatiform mole, and choriocarcinoma. It also used for checking a woman of childbearing age is pregnant before carrying out medical or surgical investigation, x-ray or drug therapy that could be harmful to an embryo.