The hook effect in pregnancy is a false-negative result. It may occur in both blood and urine pregnancy tests when someone has a high concentration of human chorionic gonadotropin (hCG), the pregnancy hormone.

Women may get a negative test result on a urine or blood pregnancy test despite being pregnant. Although many factors can contribute to a false-negative pregnancy test, the hook effect occurs when the woman has a high concentration of hCG, which oversaturates the test and results in a false negative.

Keep reading to learn more about the hook effect in pregnancy, including its causes and how to prevent it.

a woman checking a pregnancy test for the The hook effectShare on Pinterest
The hook effect may happen in a urine pregnancy test.

The hook effect occurs when blood, urine, or other samples contain too much of the substance that the specific pregnancy test is trying to detect. Instead of giving a positive result, the laboratory test becomes overwhelmed by the excess substance and provides a false-negative result.

The following substances are subject to the hook effect:

  • alpha-fetoprotein
  • CA 125
  • calcitonin
  • carcinoembryonic antigen
  • ferritin
  • follicle stimulating hormone
  • growth hormone
  • sensitive C-reactive protein
  • homocysteine
  • hCG
  • immunoglobulins
  • luteinizing hormone
  • prolactin
  • prostate-specific antigen
  • testosterone
  • thyroglobulin
  • thyroid stimulating hormone

Doctors also call the hook effect the prozone or high dose hook effect. The hook effect is uncommon. Researchers suggest that it occurs in 0.2–2% of immunoassays, which are medical laboratory tests that use antibodies to detect specific substances or analytes.

Laboratory technicians often use sandwich assays. These assays use two antibodies to detect the substance in the sample. The antibodies sandwich themselves around the substance, allowing for its detection.

When a sample contains too much of a substance, this can overwhelm the antibodies, resulting in them not attaching to it.

The test will produce a false-negative result because the antibodies are unable to detect the substance at higher-than-normal concentrations. At high concentrations, the detection signal decreases.

Doctors analyzing laboratory tests should keep the hook effect in mind because it can have important medical implications, including missed diagnoses and pregnancies.

Some women may consistently get negative urine and blood pregnancy test results despite being pregnant. In these situations, ultrasound tests may confirm the pregnancy. The hook effect is more common in the following situations:

  • ectopic pregnancies
  • twin and triplet pregnancies
  • cancer
  • other pregnancy-related diseases

Women who are experiencing symptoms of pregnancy but consistently getting negative test results from at-home urine pregnancy tests should consider speaking with a doctor. Blood tests for pregnancy may also come back negative, so the doctor may need to confirm the pregnancy with an ultrasound.

As with other diagnostic tests, doctors should make a diagnosis based on a collection of data, including symptoms, clinical findings, medical images, and laboratory testing. False-negative results from pregnancy tests can delay the confirmation of a pregnancy and, thus, prenatal care management.

It is helpful for doctors to notify people, when relevant, of the possible inaccuracy of laboratory tests and the potential for false-negative or false-positive results.

Women may have symptoms of pregnancy despite consistently receiving negative test results. During the first trimester, they may feel more fatigued than usual. Some may experience nausea and vomiting, and mood swings are also common in the first trimester.

During pregnancy, women no longer have their periods, but it is generally only within the second trimester that the belly will begin to enlarge. Women experiencing the hook effect in their pregnancy tests may still not see positive results despite their pregnancy becoming more noticeable through these changes.

When evaluating the results of a pregnancy test, doctors should consider all of the clinical signs and the laboratory and medical imaging tests. Relying on pregnancy test results may cause missed or delayed diagnoses and delayed prenatal care, which can harm both the woman and the growing fetus.

The hook effect in pregnancy occurs when a person has high concentrations of hCG, the pregnancy hormone for which both urine and blood pregnancy tests check.

High levels of hCG can overwhelm the antibodies that pregnancy tests use. When too much hCG is present in a sample, the antibodies may fail to bind, and the test will appear negative. The hook effect may occur when levels of hCG reach 500,000 milli-international units per milliliter.

Certain medical conditions, such as cancer and pregnancy-related diseases, may cause high levels of hCG. Women having twins or triplets may also have high levels of this hormone, which can be difficult for a pregnancy test to detect.

Although the hook effect occurs very rarely, the medical implications of a false-negative result can be serious. Doctors relying on blood and urine pregnancy tests may falsely assume that the woman is not pregnant. As a result, the woman may not receive proper prenatal care until an ultrasound confirms the pregnancy.

Prenatal care involves routine follow-up visits with doctors, as well as laboratory and medical imaging tests. Some people require vitamin and mineral supplements, and others may need to stop taking certain medications.

Doctors also advise making specific lifestyle changes, such as avoiding alcohol, smoking, and illicit drugs. Delaying prenatal care can be dangerous for both the woman and child.

Doctors may find it challenging to identify the hook effect in laboratory tests. Often, laboratory technicians will test undiluted and diluted samples to detect the hook effect. Although this approach can detect the hook effect in undiluted samples, it is expensive because it requires additional labor and materials.

Experts suggest that the sample pooling method can help prevent the hook effect at a much lower cost. This method involves testing a pooled sample and a 10-fold dilution of the pool.

The technician pools 10 samples from 10 people. The other samples in the batch dilute each sample 10-fold.

The laboratory worker then dilutes a portion of the pooled sample by 10-fold. The technician now has a sample that is 100-fold diluted, which they can test and compare with the pooled sample. Doctors can detect the hook effect if the 100-fold dilution sample gives a higher result than the pooled sample.

If the technician detects a hook effect, they need to reanalyze the 10 individual samples to identify which of them has a high concentration of the test substance.

The hook effect is a rare occurrence in pregnancy tests. It happens when the woman has very high levels of pregnancy hormones in her blood or urine.

The antibodies that pregnancy tests use become overwhelmed and fail to bind to the hormone. Due to this, the result comes back negative.

The hook effect can have serious medical implications because people use them to confirm pregnancies.

To confirm a pregnancy, doctors should consider the woman’s symptoms, clinical data, and laboratory and medical imaging data.