Ectopic pregnancy and the hCG discriminatory zone


I’ve written about the use of hCG testing in the evaluation of patients with suspected ectopic pregnancies.  As a reminder, the diagnostic tests used in the work-up of a woman with symptoms of an ectopic pregnancy include a combination of transvaginal ultrasound and the laboratory measurement of serum hCG concentrations.

Transvaginal ultrasound is used to visualize the fluid-filled, gestational sac in which the embryo is growing. Ultrasound may reveal 1) a normal, intrauterine pregnancy, 2) an ectopic pregnancy, or 3) neither. The latter finding would be considered an indeterminate result and occurs in about 10-30% of women being evaluated for ectopic pregnancy. An indeterminate result could represent early intrauterine pregnancy, ectopic pregnancy, or fetal loss.

These indeterminate results are often interpreted against the hCG “discriminatory zone,” the concentration of serum hCG above which a gestational sac in the uterus should be readily observed. The discriminatory zone is often considered to be between 1,500 and 3,000 IU/L.

A recent study investigated how well a serum hCG concentration ≥3,000 IU/L worked to differentiate an ectopic pregnancy from a normal pregnancy in women with symptoms of ectopic pregnancy but with indeterminate ultrasound findings. What they reported was quite interesting.

There were 141 women with an indeterminate ultrasound result and 57 of these had an hCG concentration ≥3,000 IU/L. Of these, 82% actually did have intrauterine pregnancies even though their hCG concentrations were above the discriminatory zone.

There is a very important methodological consideration to consider in this study. Pelvic rather than transvaginal ultrasonography was used and emergency department physicians and not radiologists performed it. The study purposely used pelvic ultrasound because its use in an emergency department allows for the rapid and accurate detection of an intrauterine pregnancy in the majority of cases. It also can result in a shorter stay for the patient, which, presumably, would result in lower costs. In this same study, 99% of 115 women with an intrauterine pregnancy were identified as having one using pelvic ultrasound.

The main conclusion of this study is that in women with an indeterminate pelvic ultrasound, the use of an hCG discriminatory zone does not provide information to help differentiate ectopic from intrauterine pregnancy.

The hCG Diet: Don’t waste your money


HCG dropsIt's almost predictable.  Each time I give a talk about the biochemistry of hCG and clinical uses of hCG tests, I get asked a question about the hCG diet.  A comment on a recent post on this blog also asked about its safety and effectiveness.  I decided that even though the topic is slightly outside of the realm of The Pregnancy Lab, it should be addressed here.

First some history.   In 1954, Dr. Albert T. W. Simeons, a British physician, first proposed the use of low-dose hCG injections (125 IU/day) combined with a very reduced calorie diet (500 calories/day) as a way to promote extreme weight loss.  He described his method in a manuscript titled "Pounds & Inches: A New Approach to Obesity." He believed that during pregnancy hCG promotes the mobilization and consumption of excessive fat deposits for the benefit of the developing fetus.

Several clinical studies that examined the effectiveness of the hCG diet have been performed over the years.  Tthe majority of these have clearly shown that there is no merit or truth to the claims that the hCG diet works.  While many individuals in these studies have, indeed, lost weight, there was no difference in weight loss between those who used hCG and those who used a placebo.  A meta-analysis published in 1995 concluded that the hCG diet "does not bring about weight loss or fat redistribution, nor does it reduce hunger or induce a feeling of well-being."

Many people who have tried the hCG diet will swear by it and I have no doubt that many of them have actually lost weight.  However, the weight loss is due to the ultra-low calorie diet and not to the use of hCG.  The FDA was warned consumers that hCG weight-loss products are fraudulent.

Notably, many hCG products are labeled as being "homeopathic."  Homeopathy is  a form of alternative medicine in which practitioners treat patients using highly diluted medicines.  As such, these types of hCG preparations contain undectable quantities of the hormone.  The use of this type of preparation is the equivalent to using nothing at all.  Note the "100% Hormone-Free" labeling of the hCG drops in the image above and recall that hCG is, itself, a hormone.

Perhaps most importantly, the safety of the hCG diet has recently been called into question.  Daily injections of 200 IU of hCG can be used to extend the egg maturation phase of the female menstrual cycle suggesting that the actual use of hCG for weight loss could have reproductive effects.  Prostate cells express receptors for hCG raising the concern, in men, that hCG use could lead to an enlarged prostate gland and even prostate cancer.

The lack of demonstrable weight loss effects and the possible safety concerns should be enough to make most people think twice before trying the hCG diet.  With any luck, the fad will fade away again soon.  This time, let's hope it is for good.

Four reasons for a positive hCG test in the absence of pregnancy


“Why is the pregnancy test positive if she’s not pregnant?”

This is a question I’ve been asked several times and it’s a good one.  The query usually comes from a nurse, doctor, or other healthcare provider after performing a test for human chorionic gonadotropin (hCG) and getting a positive or elevated result that they did not anticipate.

Because hCG is a hormone normally produced during pregnancy, hCG tests are used to diagnose the pregnant patient.  That makes it easy to think of hCG tests as “pregnancy tests.”  While that’s not an inaccurate label for them, identifying hCG tests as pregnancy tests gives the impression that is all they are supposed to do.  Technically, hCG tests are designed to qualitatively detect and/or measure the hormone in urine or blood and there are other reasons besides being pregnant that can cause hCG to be present.

I can think of four different reasons why hCG could be present in a non-pregnant woman.

  1. Biochemical pregnancy.  A biochemical pregnancy occurs when a woman becomes pregnant yet has a spontaneous loss of the fetus before she even knew she was pregnant.  If hCG testing occurs before all of the hCG has been metabolized out of the body then hCG can be detected by a lab test.  This situation is not as uncommon as one might think for two reasons.  First, hCG tests are frequently performed in healthcare settings in order to identify the pregnant patient in order to avoid any medical interventions that are potentially harmful to a fetus.  Second, hCG tests are capable of detecting very low concentrations of the hormone.  The high frequency of testing combined with the analytical sensitivity of the tests means that biochemical pregnancies are easily detected.
  2. Pituitary hCG.  Although the placenta normally produces hCG during pregnancy, it can be made by the pituitary gland.  The pituitary gland is a small structure in the brain that secretes many different hormones that function to regulate many endocrine organ systems.  Interestingly, three hormones normally produced by the pituitary gland (thyroid stimulating hormone, follicle stimulating hormone, and luteinizing hormone) are structurally similar to hCG.  Pituitary hCG is more commonly detected in women greater than 55 years of age but can be detected in women as young as 41 years.  Non-pregnant women with pituitary hCG usually have low concentrations of hCG present in the blood and urine.  Importantly, concentrations of hCG produced by the pituitary gland don’t show the rapid increases that occur during pregnancy.
  3. Malignancy.  Cancer cells sometimes make hCG.  While many different types of cancer have been shown to make the hormone, it’s most commonly associated with the gestational trophoblastic diseases and certain types of germ cell tumors of the testes.  Because testicular tumors occur only in men, the question of detecting hCG in the absence of pregnancy is clearly not relevant.
  4. Interfering antibodies.  Some women have antibodies in their blood that can interfere with hCG tests and cause a positive or elevated result in the absence of hCG.  Only hCG tests performed on blood can be affected by this problem because the interfering antibody molecules aren’t normally present in the urine.  This can be a serious problem because some women have been mistakenly diagnosed with cancer due to the false-positive hCG test result and have undergone unnecessary treatments for it.  The frequency of this problem is difficult to know but it’s probably very low.  Over the last several years, the manufacturers of hCG tests have worked to minimize possible interference from these antibodies but nothing can be done to completely eliminate the problem.  When alerted, the laboratory can help to determine if an hCG test result is falsely positive due to this issue.

So, just because an hCG test result is interpreted as positive doesn’t automatically mean that a woman is pregnant.  There are very valid reasons for detecting hCG in the absence of pregnancy.  That said, when the hCG test result doesn’t match the clinical picture, the laboratory should still be asked that question!  When alerted to the discrepancy, the lab can help to investigate the problem and perhaps shed some light on the cause.

There is quite a bit more to say on each of those four causes but I’ll save those comments for future posts.

 

Wanted: a rapid pregnancy test using whole blood


Most laboratorians would agree that when it comes to hCG testing for pregnancy status, a blood sample is preferred over a urine sample.  Why is that?

  • Following implantation of a fertilized egg, hCG is first detected in the blood.  hCG in the urine usually appears a day or two later.
  • Blood-based hCG test results provide a concentration of the hormone whereas urine tests are qualitative (positive or negative)
  • In early pregnancy, when hCG concentrations are still relatively low, a negative result can be obtained from a dilute urine sample (i.e. a false-negative result).

An earlier post presented the pros and cons of the different types of pregnancy tests. To summarize here: qualitative (yes/no) tests are typically easier to perform and, if urine is the sample being tested, then qualitative tests can truly be performed at the point-of-care.  If a blood sample is used, it has to be centrifuged to separate the liquid part of the blood (serum) from the solid part (blood cells), a step that is nearly always performed in the laboratory.  As such, qualitative hCG tests using a blood sample cannot be performed at the point-of-care.

A recent study challenges that idea.

Blood samples were collected from 632 women (aged 18-51 years) presenting to an emergency room. These blood sample were tested for hCG in two ways: 1) traditional, quantitative serum hCG testing in the laboratory and 2) qualitative, point-of-care hCG testing in the emergency room using the whole blood sample instead of serum.  Note that the qualitative test used was not designed nor approved by the FDA for use with whole blood and that the test results were interpreted after 10 minutes whereas the test instructions required result interpretation after 5 minutes.

When compared to the results of the quantitative test performed in the lab, the qualitative test performed on whole blood (instead of serum) performed quite well.  100% of the laboratory-negative results were interpreted as negative using the qualitative test and 96% of the laboratory-positive results were interpreted as positive using the qualitative test.  The hCG concentration of the 4% of “false-negative” results (9 out of 216) ranged from 5 to 50 IU/L.  Notably, the hCG detection threshold of the qualitative test is identified as 25 IU/L.

So, what does all this mean?  Well, the authors of the study suggest that it can be difficult to obtain urine samples from some patients which makes testing hCG in blood more attractive.  But because blood-based hCG tests are normally always performed in the laboratory, waiting for the lab results can waste time.  Hence, the appeal of testing whole blood for hCG at the point-of-care.  The results of this limited study suggest that, perhaps, whole blood hCG testing using devices that were designed for urine or serum, has merit.

What the study did not address, however, are the (often) complicated state and federal rules that regulate lab testing.  The study’s use of a qualitative hCG test using whole blood instead of serum is problematic.  Changing the type of sample recommended by the test manufacturer is a thorny issue for laboratorians who are ultimately responsible for all lab tests performed.  It is typically the lab’s responsibility to determine if an alternate sample type can be used for a given laboratory test and this type of substitution, however appealing, cannot be widely adopted without a thorough evaluation and validation by the laboratory.

Intriguing?  Yes.  Easily implemented?  Not quite.

    Surprising details about the most sensitive urine pregnancy tests


    Here’s a multiple-choice question to consider:

    Which type of qualitative urine hCG test devices (a.k.a pregnancy tests) are capable of detecting the lowest concentrations of hCG?

    A. The devices that can be purchased at a local pharmacy for use at home (over-the-counter (OTC) tests).

    B. The devices used in hospitals, clinics, and doctor’s offices (point-of-care (POC) tests).

    C. Both types of devices are equivalent.

    The correct answer is A.

    Surprised? I was too. Logic suggests that the urine hCG tests that are frequently used in healthcare settings (versus home use) would probably do best at detecting the hormone at lower concentrations. OTC urine hCG test

    There have been two studies that have investigated the analytical sensitivities of urine hCG tests. One study used samples prepared by adding hCG to male urine to achieve hCG concentrations of 6.3, 13, 25, and 50 IU/L. Fifteen different OTC pregnancy tests were evaluated, twelve of which claimed an hCG detection limit of 50-100 IU/L (the other three claimed 25 or 40 IU/L). All but one of the fifteen devices produced positive results at an hCG concentration of 25 IU/L. Curiously, the one device that did not claimed a detection limit of 25 IU/L.

    The other study (disclaimer: I was one of the investigators) used urine samples obtained from pregnant women within 10 days of their expected period. In my opinion, this is the optimal type of sample to use because pregnancy urine contains a mixture of different hCG variants and not just a single form of the hormone. That is important because not all pregnancy tests detect the same variants of the hormone equally and that, in turn, can influence device sensitivity. These urine samples were diluted to low hCG concentrations and then tested with both OTC and POC pregnancy tests. While all the devices claimed an hCG detection limit of 25 IU/L, we observed that they were capable of detecting hCG at much lower concentrations. The POC devices were sometimes four times more sensitive than claimed and the OTC devices were sometimes sixteen times more sensitive!

    The greater analytical sensitivity of the pregnancy test devices appeared to be a function of the urine sample used. For example, using the hCG Combo device, urine samples from five different women produced positive results at 25 IU/L, while samples from two women produced positive results at a lower concentration (6 and 13 IU/L) and samples from three women produced positive results at a higher concentration (50 IU/L). Likewise, the OTC devices also showed variability in analytical sensitivity with urine samples obtained from different women although all of them produced positive results at hCG concentrations below the claimed 25 IU/L detection limit. In other words, the OTC pregnancy tests gave positive results at lower hCG concentrations than did the POC pregnancy tests!

    We suspect that home pregnancy tests have greater analytical sensitivity than their POC counterparts because of differences in the amount of urine the devices require. OTC devices absorb urine from the urine stream during voiding and, as such, they use a much larger volume of urine than the tests used in healthcare settings (approximately 5-10 times more).

    To be certain, the physician who orders a urine hCG test on a patient has the same desire as the woman who performs her own urine test at home: they both want to know about pregnancy status. In the former scenario, the test is often being done to rule-out a possible pregnancy prior to a medical intervention that could harm a fetus. In the latter, the test is being done to satisfy a curiosity. Which test do you think should be the most sensitive?

    Diabetes, fetal lung maturity, and the lamellar body count


    I've written a few posts on diabetes during pregnancy lately so thought I'd stick with it a bit longer. This time, however, the focus is on the effect that maternal diabetes has on fetal lung maturity.

    It's generally believed that fetal lung maturity is delayed in women who have diabetes (gestational or otherwise). The reasons for this problem are not well understood, however high concentrations of glucose and insulin in the fetal blood have been postulated as a possible cause. Despite this, respiratory distress syndrome rarely occurs in term infants born to mothers with diabetes. Still, the issue of delayed fetal lung maturity and diabetes remains controversial.

    I'll go on record for saying that there is no valid reason to perform amniocentesis solely to determine fetal lung maturity if maternal diabetes is well-controlled. However, if mom's diabetes is poorly controlled, then there may be value in performing fetal lung maturity tests. I'd like to emphasis the "may" in that last statement. As I've written before, it seems like all tests of fetal lung maturity should become obsolete. Realistically, however, they probably aren't going to go away any time soon.

    Because the most widely used test of fetal lung maturity is about to disappear forever, many clinical laboratorians are planning to offer the lamellar body count test as a replacement. I've been asked several times what, if any, effect that maternal diabetes has on the results of this test.

    Not surprisingly, that issue has been addressed only by a few published studies. In one study from 2002, a mature result from the LBC result was believable regardless of diabetes status (note, however, that this study did not include any neonates with RDS so a true assessment of the performance of the LBC could not be made). Another study from 2009 determined that the results of an LBC test were not influenced by maternal diabetes although it did suggest that the test was still useful in preterm pregnancies and in poorly controlled maternal diabetes.

    What the take-home message? Granted, the data are limited but it's probably no more necessary to perform the LBC test in women with diabetes than it is in women without the disorder.

    hCG and the thyroid gland


    Thyroid tests First things first. The main job of human chorionic gonadotropin (hCG) hormone is to increase the synthesis of progesterone in early pregnancy. Without steadily increasing concentrations of progesterone, an early pregnancy will fail. hCG usually has nothing to do with the thyroid gland.

    As a reminder, the thyroid gland, located in the neck in close proximity to the larynx (voice box), is basically responsible for controlling our body's metabolism. It is regulated by a different hormone: thyroid stimulating hormone or TSH.

    So hCG maintains pregnancy and TSH regulates the thyroid gland. Sometimes, however, hCG can act like TSH and crank up the function of the thyroid gland. When the thyroid gland is in an over-active state the condition is called hyperthyroidism. Symptoms of hyperthyroidism include weight loss, increased appetite, heat intolerance, hair loss, weakness and fatigue, irritability, and sweating. In extreme cases, hear palpitations, and shortness of breath can occur. How can hCG cause hyperthyroidism? The answer lies in the molecular structure of these two hormones.

    As it turns out, hCG and TSH are rather similar to each other. Both are composed of two different protein subunits. One of those protein subunits is called "alpha" and the other "beta." The alpha subunits of hCG and TSH are identical but the beta subunits are a different; but not by much. The beta subunits of hCG and TSH are about 40 percent identical. When present a very high concentrations, hCG can actually stimulate the thyroid gland sending it a message to go into over-drive. In other words, hCG can sometimes act like TSH. Fortunately, this doesn't happen unless the amount of hCG in the blood gets to be very, very elevated.

    How elevated? Well, that has been the subject of some recent investigations. Conventional wisdom was that hyperthyroidism could occur in women with an hCG concentration that was greater than 50,000 IU/L. While this hCG concentration may seem very elevated, it's actually quite normal in pregnant women who are in their first trimester. Because the vast majority of pregnant women in early pregnancy do not have symptoms of hyperthyroidism, the 50,000 IU/L threshold didn't seem accurate.

    In one study (disclaimer: I participated in that study), an hCG threshold of 400,ooo IU/L was identified as the concentration above which actual symptoms of hyperthyroidism could occur. The lower hCG concentration of 200,000 IU/L was identified as the threshold above which a majority of women demonstrated biochemical signs of hyperthyroidism (i.e. decreased TSH) but they did not have actual symptoms of hyperthyroidism until the hCG increased to twice that amount.

    Another study reported that women developed suppressed TSH and/or symptoms of hyperthyroidism only when the hCG concentration was greater than 100,000 IU/L.

    A normal, singleton, intrauterine pregnancy doesn't usually produce such sustained elevations of hCG. This means that the vast majority of pregnant women will never have symptoms of hCG-induced hyperthyroidism. However, extreme elevations of hCG can be produced in women with gestational trophoblastic disease (GTD). These are a family of diseases that arise from an abnormal fertilization event so hCG is produced even in the absence of a viable fetus.

    The bottom line is that extremely high hCG concentrations can cause biochemial and physical signs of hyperthyroidism but these are rarely the result of the hCG concentrations found in normal pregnancy.

    A new way to detect gestational diabetes mellitus? Not so fast.


    In May I wrote about recommendations made by the International Association of Diabetes in Pregnancy Study Groups (IADPSG) for glucose tolerance testing in pregnancy.  Those recommendations advocate for the use of a 75-gram oral glucose tolerance test to detect gestational diabetes mellitus (GDM) in pregnant women between 24 and 28 weeks gestation and were based on findings of the Hyperglycemia and Adverse Outcomes (HAPO) study.  That study clearly demonstrated that the risks of adverse maternal and fetal outcomes continually increased as maternal glucose concentrations increased.  The American Diabetes Association adopted the IAPDSG criteria and recommends that approach to identifying women with GDM.

    In the September 2011 issue of Obstetrics and Gynecology, the American Congress of Obstetricians and Gynecologists (ACOG) opt to stick to their guns.  The ACOG continues to recommend the "two-step" approach to screen and diagnose GDM.  Step one is a screening test using either patient history, clinical risk factors, or with a 50 gram glucose administered orally followed by the measurement of blood glucose 1 hour later (a result greater than 140 mg/dL is considered abnormal).  Those with an abnormal screening test go on to have the diagnostic test: a 100 gram oral glucose tolerance test with blood glucose measured over 3 hours.

    The ACOG rightfully acknowledges that while the treatment of mild GDM decreases adverse infant outcomes there is no evidence to indicate that use of the IADPSG recommendations would result in any significant improvements in outcomes.  Of course, lack of evidence doesn't mean that there is no benefit, it simply means that any benefits have not yet been demonstrated.  The fact that widespread adoption of the IADPSG screening test would double the number of women diagnosed with GDM was also noted by the ACOG.  This increase is notable because it would lead to a considerable increase in health care costs and likely overwhelm health care delivery systems.

    In contrast to the ACOG, the National Academy of Clinical Biochemistry (NACB) have published guidelines that recommend the approach advocated by the IADPSG.  Confused yet?  Disagreement among professional groups is not unusual and consensus is not easy to come by.  As usual, a lot more research is required before widespread agreement is achieved.

    When it comes to screening for gestational diabetes, you can’t have your jelly beans and eat them, too


    I recently got a phone call from a colleague asking me what I knew about jelly beans and their use Jellybeans
    in screening for gestational diabetes mellitus (GDM). Other than being only vaguely familiar with the notion, I have to admit I didn’t know much.

    Screening for GDM is fairly straightforward and is performed between 24 and 28 weeks of gestation. The non-fasting pregnant patient drinks 150 milliliters (about 5 ounces) of a solution containing 50 grams of glucose. One hour later a blood sample is collected and blood glucose measured. If the result is greater than or equal to 140 mg/dL the screening test is considered positive and a formal oral glucose tolerance test (OGTT) is performed. Women with a positive OGTT test are diagnosed as having GDM.

    So how do jelly beans fit into all of this? Well the glucose solution that has to be consumed is sickly sweet tasting and many women find it difficult to drink. Many complain about side effects and 15 to 20 percent have nausea and vomiting (which requires re-scheduling of the test). Jelly beans offer an attractive solution to delivering a glucose load into the stomachs of moms-to-be. They are easy and fun to eat (candy at the doctor’s office!) and women report fewer unpleasant side effects when eating them compared to the glucose drink. As a bonus, their use in health care delivery systems portrays a “patient friendly” attitude (and who doesn’t want a happy patient?). Sounds like a win-win situation, right? Maybe not.

    A study of 136 women compared screening for GDM with the glucose drink and jelly beans. Regardless of the results, all of them also had the diagnostic OGTT performed. The jelly beans they used were Brach's No. 110 Jelly Beans made from sugar, corn syrup, modified cornstarch, and glucose. Based on an analysis performed by Brach, it was determined that 28 jelly beans provided 50 grams of glucose.

    Of those 136 women, 5 of them were diagnosed with GDM based on the results of the OGTT. Screening with the glucose drink identified 80 percent (4/5) of those with GDM and screening with jelly beans identified only 2 of them (40 percent). The diagnostic specificity (the percentage of truly negative results) was similar for both tests at approximately 85 percent.

    Tests that screen for a disorder like GDM should be able to detect a high percentage of women with the disease. The use of jelly beans failed to detect more than half of the women with GDM. The authors of the paper recognized that limitation and commented that if other studies found low detection rates the use of jelly beans in a GDM screening test would prevent their use for that purpose. I searched the medical literature using the terms “jelly beans” and “jellybeans” and was unable to identify any similar studies.

    When it comes to glucose challenge tests to screen for GDM, I’d lock up the jelly beans and refuse any requests to use them or any other substitute for a 50 g glucose solution.

    Determining a baby’s gender in the lab


    Male Female It's rare that I come across a pregnant woman who doesn't already know the gender of the baby she is carrying.  Ultrasound assessment of a fetus is routinely performed during pregnancy to identify fetal anomalies and, very often, gender identification is offered.  After the 13th week of pregnancy, ultrasound is quite accurate in determining a baby's sex.  There are circumstances, however, where knowing a fetus is a boy or a girl is important.  For example, male fetuses born to women who are carriers of X-linked genetic disorders are at risk of inheriting the disorder.  In such instances, knowledge of a fetus' gender before 13 weeks can be advantageous.

    Today, the Journal of the American Medica Association (JAMA) published a systematic review and meta-analysis (a systematic method of evaluating statistical data based on results of several independent studies of the same problem) that sought to determine the performance of DNA tests performed on maternal blood to determine fetal sex.  As a reminder, DNA from a fetus circulates in maternal blood.  This cell-free fetal DNA can be detected and amplified and is the basis of other emerging diagnostic tests.

    This report evaluated the results from 57 different studies that tried to determine fetal gender using a test performed on a sample of mother's blood or urine.  The 57 reports contained results from 6,541 women; 54% who delivered boys and 46% who delivered girls.  Because this was a meta-analysis, several different testing techniques were used in the included studies but all of them looked for the presence or absence Y chromosome-specific molecular markers.  If those markers were detected, the fetus predicted to be a male.  If they were not detected, the fetus was predicted to be a female.

    Collectively, the tests performed very well.  They correctly identified 95.4% of the male fetuses (test sensitivity) and 98.6% of the female fetuses (test specificity).  The tests did not work well if maternal urine was used as the test sample which is not surprising because cell-free fetal DNA is very difficult to detect in urine.  The report also noted that real-time quantitative PCR (RTQ-PCR) methods had better performance than conventional PCR techniques.

    As might be expected, test performance improved along with the gestational age of the fetus.  At less than 7 weeks test sensitivity (see above for definition) was 74.5% but increased to 95% after 7 weeks at was 99.6% at more than 20 weeks.  That makes sense because there is more fetal DNA circulating in the mom's blood as the pregnancy progresses and the fetus increases in size.

    Because of the potential clinical use of the test to identify male fetuses of mother's with X-linked genetic diseases, the authors caution that these types of sex-predicting tests should be optimized to achieve high sensitivity (detection of male fetuses) even if it compromises the specificity (detection of female fetuses).  They also cautioned that any tests developed and offered by laboratories should thorough validate that a negative (i.e. female fetus) test result reflects the presence of a female fetus and not simply the inability to detect any fetal DNA (which would also give a negative (female) result).

    While there are some direct-to-consumer labs that offer this type of DNA-based testing to determine fetal sex, the test is marketed as one of convenience to satisfy a mother's curiosity regarding her baby's gender.  Currently, I am unaware of any clinical laboratories in the United States that offer such testing for diagnostic purposes although the tests are available for that reason in the European Union.  I predict that this type of test will become more widely available in clinical labs over the next few years.  There is much promise to be realized from tests based on circulating cell-free DNA!