Hcg sensitivity of pregnancy tests

Pregnancy tests: a review - PubMed

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Review

. 1992 May;7(5):701-10.

doi: 10.1093/oxfordjournals.humrep.a137722.

T Chard¹

Affiliations

Affiliation

¹ Department of Reproductive Physiology, St Bartholomew's Hospital Medical College, London, UK.

PMID: 1639991
DOI: 10.1093/oxfordjournals.humrep.a137722

Review

T Chard. Hum Reprod. 1992 May.

. 1992 May;7(5):701-10.

doi: 10. 1093/oxfordjournals.humrep.a137722.

Author

T Chard¹

Affiliation

¹ Department of Reproductive Physiology, St Bartholomew's Hospital Medical College, London, UK.

PMID: 1639991
DOI: 10.1093/oxfordjournals.humrep.a137722

Abstract

Pregnancy tests are widely used both by the public and by healthcare professionals. All tests depend on the measurement of human chorionic gonadotrophin (HCG) in urine. Other pregnancy-specific materials have been proposed as pregnancy tests but none can better the sensitivity and convenience offered by immunoassay of HCG. Ultrasound detection is also not as sensitive as HCG measurement. The current generation of tests is based on monoclonal antibodies to the beta-subunit of HCG; these virtually eliminate the possibility of cross-reaction with pituitary luteinizing hormone (LH) and it is this feature which permits the high sensitivity. However, it is important to recognize that the 'beta-subunit' antibody reacts with both intact HCG, which is the major component in pregnancy serum, and with fragments of the beta-subunit (beta-core), which are the major form in urine. Both the blood and urine of non-pregnant subjects contain small amounts of HCG. HCG from the implanting blastocyst first appears in maternal blood around 6-8 days following fertilization; the levels rise rapidly to reach a peak at 7-10 weeks. With most current pregnancy test kits (sensitivity 25 units per litre) urine may reveal positive results 3-4 days after implantation; by 7 days (the time of the expected period) 98% will be positive. A negative result 1 week after the missed period virtually guarantees that the woman is not pregnant. With the present generation of test kits, false positive results due to interfering materials are extremely unlikely. Pregnancy tests have now reached a level of sensitivity and specificity which is unlikely to be surpassed either by better tests or alternative technology.

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Sensitivity of over-the-counter pregnancy tests: comparison of utility and marketing messages

Comparative Study

. 2005 Sep-Oct;45(5):608-15.

doi: 10.1331/1544345055001391.

Laurence A Cole¹, Jaime M Sutton-Riley, Sarah A Khanlian, Marianna Borkovskaya, Brittany B Rayburn, William F Rayburn

Affiliations

Affiliation

¹ Department of Obstetrics and Gynecology, MSC10 5580, University of New Mexico Health Sciences Center, Albuquerque 87131-0001, USA. [email protected]

PMID: 16295647
DOI: 10.1331/1544345055001391

Comparative Study

Laurence A Cole et al. J Am Pharm Assoc (2003). 2005 Sep-Oct.

. 2005 Sep-Oct;45(5):608-15.

doi: 10.1331/1544345055001391.

Authors

Laurence A Cole¹, Jaime M Sutton-Riley, Sarah A Khanlian, Marianna Borkovskaya, Brittany B Rayburn, William F Rayburn

Affiliation

¹ Department of Obstetrics and Gynecology, MSC10 5580, University of New Mexico Health Sciences Center, Albuquerque 87131-0001, USA. [email protected]

PMID: 16295647
DOI: 10.1331/1544345055001391

Abstract

Objective: To determine the sensitivity of seven over-the-counter pregnancy tests (OTC-PTs) using urine containing a mixture of human chorionic gonadotropin (hCG)-related molecules as found on the first day of missed menstrual period.

Design: Blinded in vitro sensitivity analysis.

Setting: Medical school laboratory.

Participants: None.

Interventions: Urine was tested with OTC-PT devices at titers of 100, 50, 25, 12.5, 6.3, and 0 mIU/mL hCG immunoreactivity, and laboratory workers rated their confidence in the test result based on whether the test result was a clear, sharp, and unquestionable band in the test window.

Main outcome measures: Analytical sensitivity, defined as the urine concentration at which all OTC-PTs tested gave a positive result regardless of operator confidence score; clinical sensitivity of OTC-PTs, defined as the proportion of pregnancies likely to be detected on the first day of a missed period and calculated from the analytical sensitivity and a recently published regression curve for total urine hCG immunoreactivity in 25 urine samples from this period of gestation; percentage of tested devices showing a band in the test window at a specific hCG concentration measured devices positive; percentage faulty devices, defined as the proportion of tested devices failing to yield a band in the control window; and confidence score, determined from operator ratings for each device at each concentration of hCG.

Results: First Response Early Result had an analytical sensitivity of 6.3 mIU/mL, which was estimated to detect greater than 95% of pregnancies on the day of missed period. The sensitivity of Clearblue Easy Earliest Results was 25 mIU/mL, which indicated detection of 80% of pregnancies. The sensitivity of the five other products was 100 mIU/mL or greater, indicating detection of 16% or less of pregnancies. Three of these last products included faulty devices.

Conclusion: Universal claims for OTC-PTs of more than 99% laboratory accuracy and use as early as the first day of missed period, while cleared for use by the U.S. Food and Drug Administration, are ambiguous and inappropriate for many products, according to these data. The majority of products tested were found to detect only a small percentage of pregnancies on the first day of a missed menstrual period. Until more data become available on the actual clinical sensitivity of these products, pharmacists should advise consumers to be cautious in interpreting test results.

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When to take a pregnancy test. Pregnancy test results

You can determine if you are pregnant with a pregnancy test at home. However, with all the variety of assortment, it can be difficult for a woman to decide which one to choose. Habitual to many test strip or inkjet? Is one enough or is it better to buy two at once? After all, there are many varieties of pregnancy tests today, and very different ones!

How do pregnancy tests work?

In the first days after conception, it is difficult to understand whether pregnancy has occurred, there are no specific signals. A woman can feel the first signs no earlier than the 8th - 10th day from the moment when the egg was fertilized.

After the fertilized egg is implanted in the uterine mucosa, the level of the so-called pregnancy hormone, human chorionic gonadotropin (hCG), begins to rise in the body of the expectant mother. It is produced by the chorion - one of the extraembryonic membranes of the embryo, vital for the growth and development of the fetus, which is formed starting from the seventh day of development. The chorion persists throughout pregnancy, as it takes part in the formation of a vital organ for the child - the placenta. It is made up of chorionic villi. Therefore, the hCG hormone is produced throughout pregnancy.

Normal hCG level does not exceed 5 mIU/ml. From the moment of implantation of the ovum, the concentration of this hormone doubles approximately every two to three days and reaches its peak by 7 to 12 weeks.

The effect of pregnancy tests is based on the determination of hCG in a woman's urine (a chemical reaction between hCG and a special reagent impregnated with the tester). An increased content of the hormone allows you to get a positive diagnostic result.

What types of tests are there

There are test strips, inkjet and digital pregnancy tests.

Test strip or test strip

The tester should be lowered into a container with urine up to a special mark and wait for the time specified in the instructions. The presence of two strips indicates a positive test result, one - a negative one.

It is advisable to use such a test strip in the morning, at this time the concentration of hCG in the urine is the highest, but nevertheless there are tests that can be used at any time if it has a sensitivity of no more than 10 hCG. If the option of using a urine container is not entirely convenient for you, there is a more convenient alternative - inkjet tests.

Inkjet Pregnancy Tests

They differ in a more complex device, but at the same time very convenient, which does not require an additional container for collecting urine. If it has a sensitivity of no more than 10 hCG, it can be used regardless of the time of day. It is necessary to bring the receiving part of the test system under the stream of urine for about 10 seconds. A rod is located inside the device, along which urine rises from the receiving end to the area impregnated with the reagent. The result will appear within 3-5 minutes.

Digital tests

They allow not only to find out if the pregnancy has come, but also its due date. At the same time, it is not necessary to look closely at how many stripes appeared, one or exactly two (there are often cases when the second strip is almost invisible). A clear inscription appears on the digital screen of this "diagnostician", which is stored for a day.

What to look for before buying?

The most important condition is the sensitivity of the test, its ability to respond to a particular concentration of hCG. The lower the number of hormone content indicated, the greater the sensitivity (information about this is indicated on the package).

The high sensitivity of the tests makes it possible to diagnose pregnancy already at an early stage (2-4 days before the expected date of the onset of menstruation). There is no need for repeated checks, because the accuracy of tests with a sensitivity of 10 hCG is about 99.8%, and this is 2-4 days before the expected day of delay, a test with 15 hCG will show exactly 99% no earlier than 1-2 days before the expected date of the onset of menstruation.

Most tests have a sensitivity of 20 - 25 mIU / ml (this hormone level is reached approximately 2 weeks after the intended conception). This means that you can get a reliable result only from the first day of a delay in menstruation. Up to this point, it makes little sense to do tests, since there is a high probability of getting an incorrect answer.

When choosing a test, pay attention to the number of days indicated on the package before the delay. This also affects the accuracy of testing. When used correctly, it ranges from 97 to 99%.

What can affect the accuracy of the test?

The accuracy of a home pregnancy test, and therefore its reliability, can be affected by several factors:

The time the test was performed. If taken earlier than the date indicated on the package after the date of the expected conception, it may be a false negative.

Instruction ignored. So you can miss some important nuance and get false information. Tests may differ in the method, time of application and waiting for the result.

Physiological features . If there is confidence that pregnancy has occurred (morning sickness, dizziness, frequent urination, pain in the lower abdomen and chest), and home tests do not show anything, you should consult a doctor and take a blood test for hCG (blood hormone levels increase faster).

If does not show any of the bands, it is likely that a defective test was purchased or used in violation of the instructions.

The second band is weakly visible . You need to retest in a few days. During this time, the level of hCG will increase, if there is a pregnancy, there will be a clear result. However, even a pale second strip can be interpreted as a positive test.

What else can cause a false negative or false positive result?

excessive fluid intake. This can dilute the urine, making the test less accurate.
taking diuretics, as well as medicines that contain hCG.
some pathologies of the female organs of the reproductive system (tumors).
kidney disease.

Is there an alternative to tests?

It is impossible to find out if pregnancy has occurred by any folk methods. Modern tests are the most reliable and easy way to determine if a woman is expecting a child.

The materials on the 24health.by website are for informational purposes and are intended for educational purposes. The information should not be used as medical advice. Diagnoses and prescribes treatment only your doctor. The editors of the site are not responsible for possible negative consequences resulting from the use of information posted on the site 24health.by.

When can I take a pregnancy test?

How home pregnancy tests work is based on a qualitative analysis of the level of hCG (human chorionic gonadotropin) in the urine. A reagent is applied to one of the tester strips, which manifests itself only upon contact with the “pregnancy hormone”. The presence of two strips on the test indicates a positive result of the analysis, one control strip indicates a negative reaction. According to manufacturers, the accuracy of diagnostics when using rapid tests is 97-99%. Let us analyze in more detail what factors affect the reliability of the analysis and on what day it is best to do a pregnancy test.

How soon can I take a pregnancy test?

Most pharmacy tests have a sensitivity of 25 mIU/ml. Usually, hCG reaches such a concentration in the body of a pregnant woman a couple of weeks after conception, when the fetal egg is fixed in the uterine cavity, so it makes no sense to do an analysis earlier.

With a regular menstrual cycle, diagnostics can be carried out starting from the first day of the delay. If the second strip is barely noticeable, testing is recommended to be done again the next day. The reliability of the study increases as the level of hCG increases: every day the second strip will appear faster and brighter.

To determine how long to take a pregnancy test with an irregular cycle, you need to calculate from the longest menstrual cycle in 12 months. If during this time menstruation has not begun, you can do an analysis.

For early diagnosis, an inkjet test with a sensitivity of 10 mIU/ml is suitable, which is able to determine pregnancy in a few days of delay.

When can I take a pregnancy test?

Conventional test strips in early pregnancy often show a false negative result due to daily fluctuations in the hCG hormone, so it is best to test in the morning, immediately after sleep. At this time, the highest content of hormones is observed in the urine, which increases the reliability of the study.

It is recommended to do the test before eating or drinking, so as not to reduce the level of hCG in the urine. The only exceptions are highly sensitive inkjet tests, which can be used at any time of the day without any restrictions.

Diagnostic accuracy

When testing, it is important to follow the recommendations given in the instructions for the product. This will avoid errors and increase the accuracy of the study. Manufacturers claim that the reliability of rapid tests reaches 97-99%, provided they are used correctly. False results are only possible in the following cases:

Using a tester that has expired. Pregnancy tests contain highly sensitive reagents that stain upon contact with the “pregnancy hormone”. If the device is out of date or stored in inappropriate conditions, its sensitivity is reduced, which affects the reliability of the study.
Product sensitivity is low. Standard tests with indicators of 20-25 mIU / ml often give a false negative result in early pregnancy, when the level of hCG is still too low. For maximum accuracy, the more sensitive 10 mIU/mL options are recommended. Such pregnancy tests can be done 7-10 days after the expected date of conception, even before the delay in menstruation. Sensitivity information is usually indicated in the instructions or on the product packaging.
Analysis made in the afternoon. Diagnosis is recommended to be done in the morning, when the level of hCG in the biomaterial is as high as possible. In the afternoon, the concentration of the hormone gradually drops, which increases the risk of an erroneous result.
Drinking large amounts of fluid before the examination. Water, like any liquid, dilutes urine, thereby lowering the concentration of hormones. To improve the accuracy of testing, it is recommended to do the analysis on an empty stomach.
Late assessment of diagnostic results. In the instructions for the product, the manufacturer always indicates how long the analysis will be ready. When evaluating the result before the specified period, the test may not have time to react to the “pregnancy hormone” and give a false negative reaction. The probability of error also increases when evaluating the result later than the time stated in the instructions. With prolonged contact of the biomaterial with the reagent, a barely noticeable strip often appears on the tester, which, in the absence of pregnancy, can mislead a woman.
Taking diuretics or hormonal drugs. The use of a certain group of drugs can distort the results of the diagnosis. For example, antihistamines and diuretics affect the composition of urine, making it less concentrated, which leads to erroneous test results. Conversely, taking hormones increases the chance of a false positive even when there is no pregnancy.

There are other reasons that affect the accuracy of the study. These can be tumors in the small pelvis, ectopic or missed pregnancy, physiological characteristics of the body, etc. Therefore, if you have symptoms of pregnancy and doubtful results of express diagnostics, contact your doctor for an examination.

Conclusion

Pharmacy tests are considered the most convenient method for determining pregnancy.