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Antibody Screen Positive: Rh Disease and Other Atypical Antibodies
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10. Rh Disease and Other Atypical Antibodies
Step 10. Invasive Procedures
If rising maternal antibody titers and ultrasound suggests fetal anemia, or if specialized ultrasound is not available, amniocentesis for determination of amniotic fluid unconjugated bilirubin levels (reflecting hemolysis) may be indicated. (Dugoff)
The test that is requested is the “delta OD-450”, which reflects the increasing optical density (OD), at a wavelength of 450 mu, of the amniotic fluid when it contains increasing amounts of pigment, in this case, bilirubin. The value obtained is plotted on a gestational age-based nomogram, either the 1961 Liley curve (see Diagram 1), or the newer Queenan curve (see Diagram 2). (Segatta)
http://www.uptodateonline.com/application/image.asp?file=obst_pix/queenan_.gif
These curves are divided into different zones reflecting values that suggest that the fetus is either, unaffected, affected, in an indeterminate zone, or severely affected. Amniocentesis must usually be repeated serially to see if values are rising. Formerly, a rising maternal antibody titer (more than a four-fold tube dilution) mandated amniocentesis, which then needed to be repeated every 1-2 weeks until the fetus was delivered. Testing for fetal lung maturity (with the lecithin-sphingomyelin [L/S] ratio and phosphatidyl glycerol levels) was often performed as well to see if delivery were feasible. MCA Doppler studies can now usually provide us with enough reassurance that we can follow the pregnancy without as many punctures.
Diagram 1: The Liley Curve
The Liley curve is divided into three zones.
- A result in Zone I indicates mild or no disease. Fetuses in zone I are usually followed with amniocentesis every 3 weeks.
- A result in zone II indicates intermediate disease. Fetuses in low Zone II are usually followed by amniocentesis every 1-2 weeks.
- A result above the middle of Zone II may require transfusion or delivery.
Source: Lilley Curve
ObFocus.com – Management of Isoimmunization
Diagram 2: Queenan Curve
Indian Health Staff: Go here
Not Indian Health Staff?
Queenan JT, Tomai TP, Ural SH, King JC Deviation in amniotic fluid optical density at a wavelength of 450 nm in Rh-immunized pregnancies from 14 to 40 weeks' gestation: a proposal for clinical management. Am J Obstet Gynecol. 1993 May;168(5):1370-6.
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