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Human Chorionic Gonadotropin Pregnancy Test

Pregnancy tests detect the hormone, human chorionic gonadotropin (hCG). After an egg is fertilized, the developing embryo attaches to the uterine lining approximately six to twelve days after ovulation. Following implantation, hCG is produced by trophoblastic cells that reside in the outer layer of the embryo. It takes several days for hCG to be detectable in blood or urine. Thereafter, hCG production increases very rapidly with serum concentrations doubling every 1-1.5 days during the first 8 to 10 weeks of pregnancy.

Human chorionic gonadotropin (hCG) is a glycoprotein that consists of an alpha and beta subunit. The alpha subunit is structurally similar to the alpha subunits of FSH, LH and TSH. The beta subunit is distinct for hCG. The release of hCG into maternal circulation begins with embryo implantation 5 to 7 days after fertilization. 

Several forms of hCG are present in serum and urine (Clinical Chemistry 1997; 43:2233 – 43). In early pregnancy, trophoblast cells of the placenta secrete predominantly intact hCG. This molecule is broken down into several degradation products. The normal degradation pathway is: Intact hCG- free alpha subunit & nicked free beta subunit beta core fragment

The serum of patients with uncomplicated pregnancies contains

• Intact hCG
• Nicked hCG (degradation product)
• Free alpha subunit (hyperglycosylated & not incorporated into hCG)
• Free beta subunit
• Beta core fragment

 

Urine contains all of the above plus beta core fragment. Abnormal pregancies (Down syndrome, pre-eclampsia, etc) produce much greater and more variable proportions of nicked hCG, free beta subunit, and beta core fragment.

Commercial assays use any of 7 antibody combinations that may detect:

• Only non-nicked hCG
• Non-nicked hCG & free beta subunit
• Non-nicked hCG & nicked hCG
• Non-nicked hCG, nicked hCG & free beta subunit

Beta core fragment is the predominant form of hCG in urine after 8 weeks of pregnancy but is not consistently detected by urine pregnancy tests. Urine pregnancy tests are based on a double antibody sandwich enzyme linked immunoassay that utilize a fixed capture antibody and a free-flowing detection antibody that recognize different epitopes of the hCG beta subunit. Variant hook effect occurs when one antibody with an affinity for beta core hCG becomes saturated while the other antibody does not recognize beta core hCG.

False negative pregnancy tests can result in undesirable outcomes if an inappropriate treatment is undertaken. To decrease the occurrence of false negative pregnancy test results, clinicians should request that pregnancy testing be performed on serum using a quantitative assay, especially in patients with abdominal pain, vaginal bleeding or other symptoms that strongly suggest the patient might be pregnant. Quantitative serum assays can generate results in less than an hour, can detect lower concentrations of hCG than urine tests and are not affected by hCG beta core fragment because it is not present in serum.

Another cause of falsely negative urine pregnancy tests due to the hook effect is gestational trophoblastic disease. These tumors produce extremely high levels of intact hCG. Hook effect occurs when excess hCG in the urine saturates the detection antibody and prevents formation of the antibody-antigen-antibody sandwich.

Another cause of a falsely negative urine pregnancy test is a dilute urine specimen. The lab performs a specific gravity with each urine pregnancy test. If urine is too dilute, a chartable comment is added to negative urine pregnancy test results. Unless urine is too dilute or the patient may have conceived in <10 days of testing, a urine pregnancy test is usually sufficient to rule out pregnancy.

The sensitivity of urine pregnancy tests has improved dramatically in the last several years. Current pregnancy tests can detect 12 to 20 mIU/mL of hCG in urine, depending on the manufacturer. Urine hCG concentrations reach 10 to 25 mIU/mL by 7 to10 days after conception. Urine pregnancy tests have a sensitivity of 90% to 97% and a specificity of 99% for pregnancy during the first week after a missed menstrual period.

Some over the counter pregnancy tests are more sensitive than urine pregnancy tests used in the hospital. Some home pregnancy tests can detect urine hCG concentrations as low as 5.5 mIU/L.

Urine hCG concentrations are almost always lower than serum concentrations. Most quantitative hCG assays can detect hCG at concentrations of 2 mIU/mL and some can go as low as 0.1 mIU/mL. Therefore, quantitative assays will be able to detect pregnancy several days earlier than qualitative assays.

Healthy, non-pregnant women have serum hCG levels <5 mIU/mL.  Serum hCG values >25 mIU/mL indicate pregnancy.  Levels between 5 and 25 mIU/mL often indicate early pregnancy, but results need to be interpreted cautiously because false positive results can occur in this range.  In this situation, the test should be repeated 48 hours later to confirm pregnancy.  Serum hCG concentrations double every 1.5 to 2 days during the first 6 weeks of gestation in patients with uncomplicated intrauterine pregnancies. 

Serum hCG levels can be used to estimate gestational age. Values for hCG generally peak between 8 to 12 weeks gestation and then gradually decline throughout the remainder of pregnancy.

 

Estimates may differ by as much as 2 weeks from predictions based on menstrual history.  Gestational dating by ultrasound is more accurate. 

Serum hCG levels increase in perimenopausal (41-55 years) and postmenopausal (>55 years) women, because decreases in ovarian estrogen and progesterone production result in a lessening of the negative feedback control of gonadotropin releasing hormone (GnRH) by the hypothalamus. As a result, continuous GnRH stimulation of gonadotrope cells in the pituitary leads to increased LH and FSH production as well as pituitary hCG.  Consequently, serum hCG levels as high as 8 IU/L and 15 IU/L may occur in nonpregnant perimenopausal and postmenopausal women, respectively. The hCG level should exceed 20 IU/L for pregnancy to be considered in this age group.

Quantitative serum hCG levels are helpful in evaluating suspected ectopic pregnancy because the classical triad of abdominal pain, delayed menses, and vaginal bleeding occurs in less than 20% of cases.  Most ectopic pregnancies produce sufficient amounts of hCG to test positive with urine pregnancy tests.  A urine pregnancy test can confirm that a woman is pregnant much faster and cheaper than a serum quantitative hCG.  Serum hCG quantitation can then be done to determine if a fetus is large enough to be visualized by ultrasound. Gestational sacs are visible by transvaginal ultrasonography when serum hCG levels equal or exceed 1600 mIU/mL.  If the hCG level is this high and no sac is visible in the uterus, ectopic pregnancy is suspected.  Quantitation of serum hCG levels in paired specimens drawn 48 hours apart can also be used to help diagnose ectopic pregnancy.  Women with uncomplicated pregnancies increase their serum hCG levels at least 1.6 fold during this interval.  Smaller increments are consistent with ectopic or complicated pregnancy.

The circulating half-life of hCG is 24 hours. HCG may still be detectable in maternal serum 8 to 24 days after an uncomplicated vaginal delivery.  Following a first trimester spontaneous abortion, serum hCG may be detectable up to 60 days.  

A single serum hCG level 16 days after ovulation in women who became pregnant through assisted reproductive technology provides a useful predictor of pregnancy outcome. hCG levels above 500 IU/L predict a greater than 95% chance of ongoing pregnancy.  Levels between 25 and 50 IU/L are associated with a less than 35% probability of ongoing pregnancy.  (Fertil Steril 2000;73:260-74). 

Heterophile antibodies may produce falsely elevated serum hCG results. Heterophile antibodies are human antibodies with specificity for animal antibodies used in immunoassays for hCG. The American College of Obstetricians and Gynecologists has recommended a three prong approach to rule out the presence of falsely elevated hCG due to heterophilic antibodies.

• Perform a urine pregnancy test because heterophilic antibodies are not present in urine.
• Measure hCG on serial dilutions of serum.
• Preabsorb serum to remove heterophilic antibodies.

Heterophilic antibodies are probably the cause of a persistently elevated serum hCG value of 50 mIU/mL or greater if the urine pregnancy test is negative. Measurement of hCG on serial dilutions of serum will be linear in the absence of heterophile antibodies, but nonlinear in their presence. Serum can be pretreated with a mixture of animal antibodies to absorb heterophile antibodies and prevent their interference with the hCG immunoassay. A significantly lower hCG level in the absorbed sample compare to the untreated sample, confirms the presence of heterophile antibodies.

Preoperative pregnancy testing is a controversial subject. Many anesthesia departments mandate preoperative pregnancy testing before surgery on all patients of reproductive age while others rely on the preoperative assessment to determine the need for pregnancy testing.

The American Society of Anesthesiologists (ASA) Task Force on Preanesthesia Evaluation published an advisory in 2002 that stated pregnancy testing may be considered for all female patients of childbearing age (Practice Advisory for Pre-Anesthesia Evaluation. Anesthesiology 96:485-496, 2002). Clinical characteristics to consider included an uncertain pregnancy history or a history suggestive of current pregnancy. The Task Force recognized that a history and physical examination may be insufficient for identification of early pregnancy.

An updated Practice Advisory for preanesthesia evaluation was published in March 2012. ASA reiterated that the literature was inadequate to inform patients or physicians on whether anesthesia causes harmful effects on early pregnancy. Pregnancy testing may be offered to female patients of childbearing age and for whom the result would alter the patient's management.

In 2013, as part of the Choosing Wisely campaign, ASA reiterated that they did not include pregnancy testing in their top 5 preoperative recommendations of tests to avoid because physicians and hospitals already had the opportunity to set their own practice and policy (October 12, 2013, www.choosingwisely.org).Thus, ASA does not require preoperative pregnancy testing.

A review of the literature reveals that the incidence of detecting a previously unrecognized pregnancy preoperatively ranges from 0.2 to 1.2%. One of the earliest studies that tested women of childbearing age within six days of ambulatory surgery in Chicago detected 7 previously unrecognized pregnancies out of 2056 women tested for an incidence of 0.3%.

Most preoperative testing for pregnancy has relied on qualitative urine hCG testing using a point of care device. These tests were originally designed to confirm pregnancy after a missed expected menstrual period. These devices generally have cutoffs for positivity at 20, but sensitivity may vary by brand. Because of this limited sensitivity, first morning urine samples are recommended because they are the most concentrated of the day. Urine hCG tests were not intended for preoperative detection of early pregnancy on a random urine sample. Early pregnancies are most likely being missed using this testing strategy.

Quantitative hCG assays performed on blood samples are much more analytically sensitive than qualitative urine assays. Most quantitative hCG assays can detect hCG at concentrations of 0.1 to 2.0 IU/L. Also, hCG concentration is more than twice as high in serum than urine and serum concentration is not affected by fluid intake. Therefore, quantitative assays detect pregnancy earlier and more reliably than qualitative urine assays. Quantitative serum hCG assays are better designed for preoperative detection of early unrecognized pregnancy, but are much more expensive. The cost benefit ratio of preoperative testing with a quantitative assay needs to be calculated.

Based on review of available literature, the following conclusions are warranted:

1.     Routine preoperative pregnancy testing for all female of childbearing age remains controversial.

2.     According to the ASA, preoperative pregnancy testing is optional.

3.     If pregnancy testing is offered, informed consent should be obtained from competent patients and/or parents.

4.     Urine hCG has been the test of choice preoperative testing, but is not optimal.

5.     Quantitative hCG testing of serum would be more sensitive but significantly more expensive.

Urine pregnancy tests are reported as positive or negative.  Reference value is negative.

Serum hCG reference range is:

 

Specimen requirement for the urine pregnancy test is 10 mL of a first morning midstream urine collection in a urine specimen cup.  Specimen requirement for quantitative serum hCG is one SST tube of blood.

References

ACOG Committee on Gynecologic Practice, Avoiding inappropriate clinical decisions based on false positive human chorionic gonadotropin test results. Reaffirmed 2017, ACOG Committe Opinion No. 278.

Manely et al. Preoperative Pregnancy Testing in Ambulatory Surgery: Incidence and Impact of Positive Result Anesthesiology vol 83 (4), October 1995, 690-693.

Latifi N, et al. Point-of-Care Urine Pregnancy Tests. JAMA, published online November 11, 2019.