Pregnancy Tests: How hCG Detection Works (and Why "No Clumping" Means Pregnant)

Most of the laboratory tests for pregnancy are based on detecting the presence of human chorionic gonadotropins (hCG) in the woman’s urine. The hCG (human chorionic gonadotropin) is secreted by syncytiotrophoblast cells of the placenta. hCG appears as early as the sixth day after fertilization in blood and the eighth day after fertilization in urine, the peak is reached at the ninth week of pregnancy. The level of HCG sharply decreases during the fourth and the fifth months.

Most modern pregnancy tests are immunoassays that detect hCG through an antigen-antibody reaction. Several use a clever inverted form of the agglutination test called agglutination inhibition, where the absence of clumping, not its presence, signals a positive result. Understanding that inversion is the key to reading these tests correctly.

HCG hormone prevents the degeneration of the corpus luteum (which occurs about two weeks after ovulation) and maintains its viability. This allows continued secretion of estrogens and progesterone by the corpus luteum, which are required for the prevention of menstruation and continued attachment of the fetus to the uterine endometrium.

Some pregnancy tests are so sensitive that the earliest pregnancy diagnosis can be made within a few days of conception, i.e., even before the next missed period. These tests can be grouped into biological, immunological, and radiological.

Biological Diagnostic Tests

These tests are now obsolete and of historical interest only, replaced entirely by immunoassays. They are worth knowing for context and exams, not for practice.

Biological tests for pregnancy detection involve the injection of urine into various animals. Though these test methods are 99% accurate, they are not routinely performed in clinical practice as they are time-consuming and expensive. Another major limitation is the requirement of knowledge of the histological study of gonadal tissues for test result interpretation.

Some popular biological pregnancy detection tests are the Aschheim-Zondek mouse test, Friedman rabbit test, Galli Mainini frog test, and Hogben test.

Immunological Tests

Immunological tests are used routinely in all hospitals and clinics to detect pregnancy. Compared to biological tests, they are less costly, simpler, easy to carry out, take very little time for reporting, and can confirm pregnancy within ten days of conception.

Figure: Happy woman holding pregnancy test stick in the bathroom.

Principle

The HCG (human chorionic gonadotropins) secreted by syncytiotrophoblast cells of the placenta is antigenic. Antibodies against this hormone are available commercially and are employed to detect the presence of hCG in the serum or urine of pregnant women. HCG can be detected using various immunological methods such as precipitation, hemagglutination, complement fixation test, etc.

hCG is a two-subunit hormone. Its alpha subunit is almost identical to the alpha subunit of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH), while its beta subunit is unique to hCG. This matters enormously for test design. An antibody directed against the shared alpha subunit will cross-react with LH and can give a false positive, especially around the LH surge at ovulation or at menopause when LH is high. That is why modern pregnancy tests use a monoclonal antibody against the beta subunit of hCG: targeting the unique subunit is what makes the test specific for pregnancy rather than for pituitary hormones. When you read later that a raised LH can cause a false positive, this is the reason, and it is also the reason good kits avoid it.

The slide (latex) tests are relatively insensitive, detecting hCG at roughly 1.5 to 3.5 IU/mL, while tube tests are more sensitive, detecting around 0.2 to 1.0 IU/mL. The more sensitive the test, the earlier in pregnancy it can give a reliable result. This concentration is reached 10th day of fertilization (i.e., even before the missed period). A pregnancy test can be performed anytime after the eighth day of pregnancy and is ideally performed after 14 days of missed period.

Procedure

Women undergoing pregnancy tests are advised to restrict water intake for 12-14 hours. They can restrict water intake from the evening and collect the first urine sample the following day in a clean container.

A pregnancy test should be performed within 12 hours of urine collection. The specific gravity of urine should be at least 1.015 and free from protein and blood.

Latex Agglutination Inhibition (LAI) Test (Gravindex test)

The latex agglutination inhibition test is the classic example of a test where no clumping is the positive result, and the logic is worth walking through slowly because it reverses everything you expect from an agglutination test.

The kit contains two reagents: anti-hCG antibody, and latex particles coated with hCG. The patient's urine is mixed with the antibody first, then the coated latex is added.

If the woman is pregnant, her urine contains hCG. That hCG binds and uses up the anti-hCG antibody. When the hCG-coated latex is then added, there is no free antibody left to cross-link it, so no clumping occurs. No clumping means pregnant.

Figure: Latex agglutination inhibition test for pregnancy detection

If the woman is not pregnant, her urine has no hCG. The anti-hCG antibody remains free, so when the hCG-coated latex is added, the antibody bridges the particles and they clump. Clumping means not pregnant.

The rule to hold onto: the analyte you are testing for (hCG) works by blocking the visible reaction, so its presence is read as the absence of agglutination. This is what "inhibition" means in the test's name.

Hemagglutination Inhibition (HAI) Test (Prognostican test)

In this test, sheep’s RBCs coated with hCG are employed in place of latex particles. The test is done in a test tube, and observations are made after 2 hours. The principle is the same as LAI. A positive pregnancy test is indicated by the formation of a sharply demarcated brown ring at the bottom of the tube; the absence of the ring suggests a negative test.

One-step Immunoassay test

This test is based on combining monoclonal antibody-dye conjugate with polyclonal solid-phase antibodies to qualitatively detect hCG in the urine.  A urine sample is applied to the test zone of the card or strip, and if it contains hCG, a pink-purple-colored band develops. A control is provided to check the potency of the test reagents. Several test kits under different proprietary names are available.

Figure: ICT-based test for the detection of pregnancy (Image source: Andy Brunning)

A urine pregnancy test can be performed at home or in a clinic. Depending on the test format, the test kit can be dipped into the urine (dipstick urine pregnancy kit), or urine is applied using an eyedropper in the sample window. Depending on the test format, a color change, the appearance of the test line, a symbol (like a plus or minus), or pregnant or non-pregnant words denote the test result.

This is a lateral flow immunochromatographic test: the urine carries hCG along a strip where a labeled anti-hCG antibody binds it, and the complex is captured at a test line to produce a visible band. Unlike the agglutination inhibition test, here a visible line (not its absence) is the positive result, which is why home tests are easier to read.

Radioimmunoassay (RIA)

Radioimmunoassay is a more sensitive pregnancy test method and can detect the presence of hCG in the serum as early as 7-10 days following fertilization.

hCG radiolabeled with iodine-125 is mixed with a fixed amount of anti-hCG antibody and the patient's sample, which compete for antibody binding (a competitive immunoassay). RIA can detect as little as 0.003 IU/ml of the β subunit and 0.001 IU/ml of the α subunit of HCG in the urine or serum.

ELISA (Enzyme-linked immunosorbent Assay)

ELISA has been widely used to detect a variety of antigens and antibodies. The principle of the test is the same as that of RIA, except that an enzyme is used in place of a radioactive substance. The enzyme acts on the substrate to produce a blue color, a positive pregnancy test.

Although immunological pregnancy tests are sensitive and accurate, false-positive and false-negative results can occur.

False Positive Results

False positive pregnancy results (pregnancy test is positive, but the woman is not pregnant)*may be obtained due to excessive protein or blood in the urine sample, at menopause, or at the time of ovulation due to increased secretion of luteinizing hormone (LH). Such results can also be seen at the beginning or with malignant tumors of the placenta in which hCG is produced from other sources like choriocarcinoma and hydatidiform mole. Thiazide diuretics, steroids, and thyroid drugs may affect the outcome of early pregnancy tests.

False Negative Results

False negative pregnancy results (pregnancy test is negative, but the woman is pregnant) may be seen when the concentration of hCG in urine or serum is very low or testing too soon, or due to an ectopic pregnancy.

Ultrasonography

Ultrasonography (ultrasound) is a real-time, non-invasive pregnancy detection method and the most reliable method of detecting pregnancy. Pulses of ultrasonic waves at high frequency are generated from a piezo-electrical crystal transducer that also acts as a receiver to detect waves reflected from various parts of the uterus. The echoes (reflected waves) are displayed on the ultrasound screen.

Figure: Pregnant woman on ultrasound

The gestational ring is evident as early as the 5th week of pregnancy, cardiac pulsations by the 10th week, and fetal movements by the 11th week. Ultrasonography is particularly useful in detecting fetal viability and position, site of the placenta, multiple pregnancies, amount of liquor present,  fetal-maternal abnormalities, etc. The method is also used for the determination of the sex of the fetus.

How to Remember

Inhibition inverts the answer. In an ordinary agglutination test, clumping is positive. In the pregnancy agglutination inhibition test, the hCG you are hunting blocks the clumping, so no clumping is positive. Whenever a test has "inhibition" in its name, expect the result to read backwards.

Beta is the pregnancy-specific half. hCG shares its alpha subunit with LH, FSH, and TSH, but its beta subunit is unique. Tests aimed at beta are specific for pregnancy; tests that catch the shared alpha can be fooled by an LH surge. Beta means pregnancy, alpha means confusion.

Key exam facts in one table

Where students get confused

"Clumping means the test is positive." Not in the latex agglutination inhibition pregnancy test. There, hCG uses up the antibody so the latex cannot clump, meaning no clumping is the positive (pregnant) result. This is the single most confusing point in the whole topic, and it is a favorite exam trap.

"A positive pregnancy test always means pregnancy." hCG, or something that looks like it, can rise without a normal pregnancy. Trophoblastic tumors (hydatidiform mole, choriocarcinoma), a recent hCG injection, and a high LH surge can all produce a positive. A positive test means hCG is present, which usually but not always means pregnancy.

"Why do some tests give false positives at menopause or ovulation?" Because LH is high at those times, and LH shares the alpha subunit with hCG. A test using antibody against the shared alpha subunit can be fooled. Modern beta-subunit tests avoid this.

"A negative test rules out pregnancy." Not if it was done too early, before hCG has risen, or if the urine was very dilute, or in an ectopic pregnancy where hCG rises abnormally. A negative test in someone with a missed period should be repeated in a few days.

"The home test and the lab latex test work the same way." They are opposite in reading. The home lateral flow test shows a visible line when hCG is present (positive). The latex agglutination inhibition test shows no clumping when hCG is present. One reads presence as a signal, the other reads presence as the absence of a signal.

References and further readings

• What is hCG? American Pregnancy Association. https://americanpregnancy.org/getting-pregnant/hcg-levels/
• Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
• Gnoth, C., & Johnson, S. (2014). Strips of hope: accuracy of home pregnancy tests and new developments. Geburtshilfe und Frauenheilkunde, 74(7), 661–669.
• Betz, D., & Fane, K. (2023). Human chorionic gonadotropin. In StatPearls. StatPearls Publishing.