cyra
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A 31-year-old Caucasian woman who is in her 19th week of gestation comes to the office to discuss the results of a slightly elevated serum AFP. She has been followed since her seventh week of pregnancy. Ever since she learned that women must start taking folic acid before getting pregnant in order to avoid congenital defects, she has been concerned about the possibility of having a baby with neural tube defects. Because she just started taking folic acid during her second month of pregnancy and one of her friends had a child with spina bifida, she requested her alpha-fetoprotein (AFP) levels to be tested. She has no other medical problems. She does not use alcohol, tobacco or illicit drugs. Her family history is not significant. She is currently not taking any medications, except for folic acid. The physical examination reveals clear lung fields, and normal first and second heart sounds. The abdomen is soft and non-tender. The uterine size, fetal movements and fetal cardiac activity are all normal. Neurologic examination reveals no abnormalities. Her serum AFP level is slightly elevated, reaching 2.7 MoM (normal values: less than 2.5 MoM). Which of the following is the most appropriate next step in management?
A. Reapeat the serum AFP test
B. Perform amniocentesis for chromosomal analysis
C. Perform amniocentesis and evaluate amniotic fluid AFP levels
D. Order an obstetric ultrasound
E. Order amniotic fluid acetylcholinesterase (AChE)
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| vradojc1
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First thing to do is to assess accurate dating of pregnancy - so it would be D. If dates are OK, proceed with amniocentesis for AFP and ACHesterase (C+E).
In chromosomal abnormalities (Down) MSAFP is low so no need for B. You would do A if the dates were incorrect (after US) and you are still in the 15-20 wk period so you can proceed with amnio if MSAFP is abnormal.
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| drk1980
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Since its mentioned quite clearly that this woman has been good with prenatal care and GAge is documented....even though its part of protocol, will Ultrasound not be redundant in this case? Moreover, she started FAcid only in the second month....which means she could be at increased risk.
I remember doing this Q....but blanked out on the answer/expln!
I am guessing C or E. Sorry couldnt provide a better opinion!
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| coolmavs
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Agree with drk1980, but I think that the right answer is C and not E, since ACh'esterase will only be elevated in case of OPEN ( and NOT Closed) NTD while increased AFP can also be cause of closed NTD.
BTW, is it not possible to get any clue about NTD from USG.
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| vradojc1
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I'd pick D even in this case. Errors in dating using US are still possible, although if done prior to 12th week minimal (+/- ~ 1 week). However that slight error can result in discordance of findings.
Then again, she is at the end of time when therapeutic abortion can be performed and maybe amniocentesis with AChE would be better - it is more specific for NTDs than AFP.
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| vradojc1
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From UpToDate
Screening protocol — MSAFP screening at 15 to 20 weeks of gestation should be offered to all pregnant women as it is an effective method for detecting NTDs [15]. AFP screening is primarily intended for the detection of open spina bifida and anencephaly, but can also uncover several nonneural fetal abnormalities (eg, ventral wall defects, tumors, dermatologic disorders, congenital nephrosis, aneuploidy). It does not detect closed spina bifida. (See "Pregnancy complications predicted by second trimester maternal serum screening").
Ultrasound examination — Ultrasound is an effective technique for detecting NTDs, and can potentially detect more NTDs than MSAFP [44]. The diagnosis of anencephaly is based upon the absence of brain and calvarium superior to the orbits on coronal views of the fetal head. The sonographic diagnosis of this condition is highly accurate and should not be missed on any routine second or third trimester ultrasound examination. The sensitivity of sonographic diagnosis of other NTDS is high, but depends in part upon the size and location of the defect, the position of the fetus, the volume of amniotic fluid, maternal habitus, and the skill and equipment of the sonographer. (See "Ultrasound diagnosis of neural tube defects").
Amniocentesis — As discussed above, ultrasound examination may be diagnostic of a NTD. However, ultrasound findings may be uncertain or show an apparently normal fetus. In these cases, further evaluation is usually indicated. Amniotic fluid AFP (AFAFP) and amniotic fluid acetylcholinesterase (AChE) are the primary biochemical tests performed on amniotic fluid for detection of open neural tube defects. AChE is an enzyme contained in blood cells, muscle, and nerve tissue. An elevation of both AFP and AChE values suggests an open fetal NTD with 96 percent accuracy and a false positive rate of 0.14 percent [3]. Blood contamination of the amniotic fluid sample is responsible for one-half of false-positive AChE results. (See "Amniocentesis: Technique and complications")
Sonographic versus amniotic fluid diagnosis — Some authors have suggested that the rate of detection of NTDs by ultrasound examination alone may preclude the need for amniocentesis [45-48]. These findings are illustrated by the following examples.
* In one study of over 2000 women with an elevated MSAFP, sonography alone was 97 percent (66 of 68 cases) sensitive and 100 percent (2189 cases) specific in diagnosing an open NTD [46]. Suspicious findings on sonography led to an amniocentesis for confirmation of NTD in the two cases of NTD not specifically diagnosed on ultrasound examination.
* A second series of 905 pregnancies found that 49 neural tube defects were correctly diagnosed by ultrasound alone; one was not [47]. The sensitivity, specificity, and positive and negative predictive values of ultrasound evaluation for the detection of NTDs were 98, 100, 100, and 99.9 percent, respectively. Forty-three other abnormal fetuses were also detected in patients with an elevated MSAFP, including 19 with abdominal wall defects, seven with findings suggestive of chromosomal abnormalities, five with urinary tract abnormalities, one with a cardiac abnormality, and 11 others; two fetuses with chromosomal abnormalities were not detected. The authors felt that ultrasound could be used reliably to detect NTDs, thereby avoiding the risks of amniocentesis.
* A review of the ultrasound findings in 51 consecutive fetuses with pathologically confirmed spina bifida, encephalocele, gastroschisis, or omphalocele calculated the sensitivity of ultrasonography for these diagnoses and the probability of an affected fetus in women with a given level of MSAFP and a normal second trimester sonogram at their facility [45]. These four types of anomalies were correctly identified in all 51 cases, yielding a sensitivity of 100 percent. The probability of an affected fetus ranged from 0.01 to 0.15 percent for MSAFP levels ranging from 2.0 to 3.5 MoMs, respectively. The authors concluded that this level of risk was less than the reported risk of a procedure related spontaneous abortion after amniocentesis (0.3 to 0.5 percent) and, therefore may lead some women with an elevated MSAFP to decide not to proceed with amniocentesis.
* Cost-benefit analyses have revealed savings of approximately $36 to $49 million dollars in annual savings if ultrasound examination replaced amniocentesis for the diagnosis of NTD [49].
In contrast, a series of 161 cases of open spina bifida identified by the California Maternal Serum Alpha-Fetoprotein Screening Program reported 8 percent of NTDs were not diagnosed by the initial ultrasonographic evaluation and three defects were not recognized until birth [50]. The authors concluded that ultrasonography was not sufficiently sensitive to forego amniocentesis. Small encephaloceles and spina bifida may be missed with ultrasonography and factors cited above, such as large maternal body habitus, fetal position, and lack of experience of the sonographer contribute to the difficulty of sonographic diagnosis [50,51].
Summary — Based upon review of existing data, the most sensitive approach to the prenatal diagnosis of NTDs is MSAFP screening followed (if the MSAFP is elevated) by a combination of ultrasound examination and amniocentesis [15]. Detection rates for NTDs are greater than 95 percent with rare false positives with both of these modalities. If one is certain of the diagnosis based upon the elevated MSAFP level and the ultrasound findings, an amniocentesis for confirmation may not be necessary. However, if there is any uncertainty about the diagnosis, or if the patient wishes to find out the karyotype (given the association with chromosomal abnormalities) an amniocentesis is warranted. The information may be particularly useful for diagnosis and estimating recurrence risk if there are associated anomalies.
We, and others, feel that a fetal karyotype should be obtained at the time of amniocentesis since this test adds no additional risk to the procedure, there is an elevated risk of chromosomal abnormalities with NTDs, and this information assists in accurate diagnosis of the current pregnancy and may be important for counseling regarding recurrence risks [30,52]. However, other authors have not recommended chromosome analysis in women under 35 years of age in the presence of a normal sonogram because of the cost and low risk of abnormality (0.3 to 0.6 percent) [53,54].
MSAFP results are expressed as multiples of the median (MoM) for each gestational week because these values are easy to derive, more stable, and allow for interlaboratory variation. The median value, rather than the mean, is used because it is not influenced by occasional outlying values. A value above 2.0 to 2.5 MoM is designated an abnormal result, depending upon the laboratory's preference for balancing the detection and false-positive rates in their population.
A first elevated test may be repeated because as many as 30 percent of moderately elevated MSAFP results will be below the threshold level upon repeating the test and such findings are not associated with an increased frequency of false-negative NTD diagnoses [37]. If the elevation persists, then the next step is to obtain a specialized ultrasound examination to further assess whether a NTD, or other anomaly, is present [15]. An advanced gestational age, patient anxiety, or a significantly elevated value may preclude repetition of the test, in which case sonography should be obtained expeditiously.
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| coolmavs
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I stand corrected, I think that you would do a USG but:
1. Since it is NON-INVASIVE and less risk.
2. To check for any congenital malformation, which may cause increased AFP.
3. Just to reconfirm that there is no dating error.
But then since MSAFP is only a borderline elevation you can also go in for a repeat MSAFP.........
What is the answer cyra?
Edited by coolmavs on 04/23/07 - 08:49 AM. Reason: typo
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| vradojc1
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2.5 MoM is not borderline - it is elevated
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| usmle12
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it as simple no need to go into details.....most comon cause of elevated MSAFP is dating errors so ultrasound comes before any thing and kaplan notes explicitly tells that
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| Justice
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D. Order an obstetric ultrasound
I have an addition to other comments, that comes from personal experience. In US, all women at 20 week of gestation are given US for spina bifida, regardless AFP level... The woman is right about to have her US test...
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| cyra
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I am so sorry guys...for soem reason the site just wouldn't let me log in these past few days.
This question was weird because the answer given there was A. Heres the explaination.
Serum AFP levels are usually measured between 16-20 weeks of gestation to detect open neural tube defects. An elevated AFP value (greater than 2.0 to 2.5 MoM) is present in open spina bifida and anencephaly; however, this may also be present in other fetal abnormalities, such as congenital nephrosis, ventral wall defects, dermatologic disorders, tumors, etc. This test does not rule out all cases of spina bifida because it is unable to detect cases of closed spina. Studies have shown that elevated maternal AFP levels are 91% sensitive and 96% specific for spina bifida, and 100% sensitive for anencephaly; therefore, if the AFP levels are elevated but not greater than 7.0 MoM, the test must be repeated to prevent overdiagnosis. In up to 30% of these repeat tests, the results will be normal.
(Choice D) An obstetric ultrasound will be needed if the repeated values are high, in order to confirm the gestational age and to search for any evident neurological abnormalities. A pelvic ultrasound should be done without repeating the measurement of AFP levels only if there is a strong family history for neural tube defects, or if the AFP levels are higher than 7.0 MoM.
(Choice C) Amniocentesis should only be performed when the sonogram fails to show any neural tube defect. Amniotic fluid AFP levels can then be measured, and elevated values are diagnostic. This patient has not had an ultrasound. For this reason, evaluation of the amniotic fluid AFP is not the most appropriate next step in management.
(Choice E) Acetylcholinesterase (AChE) is contained in neural tissue, blood cells and muscle. Measurement of the amniotic fluid AChE levels is also performed to detect the presence of any open neural tube defects. It is usually measured at the same time as amniotic fluid AFP (after an ultrasound has been performed).
(Choice B) Chromosomal analysis has the objective of ruling out Down's syndrome, which is usually associated with decreased, not increased levels of AFP. Hence, it is not helpful for this patient.
So,if AFP is elevated but is <7 --> repeat test -->if value still high -->proceed wth USG to rule out dating error ad take it from there.(which is what vradojc1's post says)
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| webjeee
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I didn't find in my uw
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