Things to know about the hook effect in pregnancy

Things to know about the hook effect in pregnancy

In pregnancy, the hook effect is a false-negative result. It is a phenomenon that may occur in pregnancy tests of both blood and urine.

Despite being pregnant, women may get negative test results on a urine or blood pregnancy test. Although many factors may contribute to a false-negative pregnancy test, the hook effect occurs when the woman has a high concentration of the pregnancy hormone, the human chorionic gonadotropin (hCG). The high levels are oversaturating the test and leading to a false negative.

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

What is the hook effect?

A lady holding hook effect
In a urine-pregnancy test, the hook effect may occur.

The hook effect occurs when blood, urine, or other samples contain too much of the substance attempted to detect by the specific pregnancy test. Instead of yielding a positive result, the excess substance overwhelms the laboratory test and provides a false-negative outcome.

The following substances undergo the effect of the hook:

  • 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 prozone or high-dose hook effect a hook effect. The hook effect is rare. Researchers suggest it occurs in 0.2–2 percent of immunoassays, which are medical laboratory tests using anticorps to detect specific substances or analytes.

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

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

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

The hook effect should be kept in mind by doctors analyzing laboratory tests because it can have significant medical implications including missed diagnoses and pregnancies.

How does it relate to pregnancy?

While pregnant, some women may consistently get negative urine and blood test results. The pregnancy may be confirmed by ultrasonic tests in these situations. In the following situations the Hook effect is more common:

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

Women who experience pregnancy symptoms but consistently get negative test results from urine pregnancy tests at home should consider talking with a doctor. Blood tests for pregnancy may also come back negative, so the doctor may need an ultrasound to confirm the pregnancy.

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

Doctors are helpful when notifying people of the possible inaccuracy of laboratory tests and the potential for false-negative or false-positive results where relevant.


Women may have pregnancy symptoms despite getting consistently negative test results. They may feel more fatigued than usual over the first trimester. Some may experience nausea and vomiting, and in the first trimester, mood swings are common too.

Women no longer have their periods during pregnancy, but generally the belly will only begin to enlarge within the second trimester. Women experiencing the hook effect in their pregnancy tests may still not see positive results despite those changes making their pregnancy more noticeable.

Doctors should consider all clinical signs, and laboratory and medical imaging tests when evaluating the results of a pregnancy test. Based on the results of the pregnancy test, missed or delayed diagnoses and delayed prenatal care can cause damage both to the woman and to the growing fetus.


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

High levels of hCG can overwhelm the anticorps used in pregnancy tests. When a sample contains too much hCG, the antibodies may fail to bind, and the test appears negative. The hook effect can happen when hCG levels reach 500,000 mili-international units per milliliter.

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


Although the hook effect occurs very rarely, the medical consequences of a false-negative outcome can be serious. Doctors who rely on blood and urine pregnancy tests may mistakenly assume the woman is not pregnant. As a result the woman may not receive proper prenatal care until the pregnancy is confirmed by an ultrasound.

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

Doctors also advise you to make specific changes to your lifestyle, such as avoiding alcohol, smoking and illicit drugs. Delaying prenatal care can be dangerous for women and children alike.


Doctors might find identifying the hook effect in laboratory tests challenging. Laboratory technicians will often test undiluted and diluted samples for hook effect detection. While this approach may detect the hook effect in undiluted samples, it is expensive because additional labor and materials are required.

Experts suggest that the method of sample pooling can help to prevent the hook effect at far lower cost. This method involves testing a pooled sample and having the pool 10 times diluted.

The technician pools 10 samples out of 10 persons. The other batch samples dilute every sample by 10 times.

A portion of the pooled sample is then diluted by 10 times by the laboratory worker. Now, the technician has a 100-fold diluted sample, which they can test and compare with the pooled sample. If the 100-fold dilution sample gives a higher result than the pooled sample, doctors can detect the Hook effect.

If a hook effect is detected by the technician, the 10 individual samples must be reanalyzed to identify which one of them has a high concentration of the test substance.


The Hook effect in pregnancy tests is a rare occurrence. It happens when the woman’s blood or urine contains very high levels of pregnancy hormones.

The antibodies used during pregnancy tests get overwhelmed and fail to bind to the hormone. Because of this the result returns negative.

The hook effect can have severe medical consequences because people use it to confirm pregnancies.

Doctors should consider the woman’s symptoms, clinical data, and data from laboratory and medical imaging to confirm a pregnancy.


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