Amniotic fluid

5 DOH Abu Dhabi (HAAD) lab licensing practice questions on Amniotic fluid, part of Urinalysis and Body Fluids. The first 5 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   At 31 weeks, amniotic fluid from a pregnancy affected by maternal anti-D is scanned. The absorbance at 450 nm is 0.38, and the baseline drawn from 365 to 550 nm reads 0.20 at 450 nm. Using the chart shown, the ΔA450 and its zone are:

   

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   1. A0.18, in zone II
   2. B0.18, in zone III
   3. C0.58, in zone III
   4. D0.38, in zone III

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   Answer: A · 0.18, in zone II

   ΔA450 is the height of the bilirubin hump above the baseline: 0.38 - 0.20 = 0.18. On the chart at 31 weeks, the zone II/III line lies at about 0.24 and the zone I/II line at about 0.06, so 0.18 falls in zone II, indicating moderate hemolysis. Zone II results are repeated to see whether the trend rises toward zone III, where intrauterine transfusion or delivery is considered. Middle cerebral artery Doppler has now largely replaced amniocentesis for this purpose, but the measurement principle is the same.

   • B: At 31 weeks the zone III line is near 0.24, above 0.18; this value reaches zone III only after about 35 weeks.
   • C: The baseline reading was added instead of subtracted.
   • D: 0.38 is the raw absorbance; the baseline must be subtracted.

2. Question 2 · Medium

   The sharp peak near 415 nm on the amniotic fluid scan shown is due to:

   

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   1. ABilirubin from fetal red cell breakdown
   2. BPhoto-oxidation of bilirubin by light
   3. COxyhemoglobin from blood in the sample
   4. DLamellar bodies from the fetal lungs

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   Answer: C · Oxyhemoglobin from blood in the sample

   Oxyhemoglobin has a strong absorbance peak at about 410-415 nm, with small extra bands near 540 and 576 nm, which are also visible on this scan. Bilirubin produces the broader hump at 450 nm, and the overlapping hemoglobin peak inflates the reading there, so a blood-contaminated fluid can give a falsely high ΔA450. The contamination is reported and the result interpreted with caution. Fluid for bilirubin scanning is also protected from light, which would lower the 450 nm hump.

   • A: Bilirubin gives the broad hump centered at 450 nm, not the sharp peak at 415 nm.
   • B: Light destroys bilirubin and flattens the 450 nm hump; it does not add a peak.
   • D: Lamellar bodies scatter light and are measured as turbidity at 650 nm, with no sharp peak.

3. Question 3 · Easy

   At what wavelength is the rise in absorbance used to estimate bilirubin in amniotic fluid?

   1. A450 nm
   2. B340 nm
   3. C540 nm
   4. D410 nm

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   Answer: A · 450 nm

   Bilirubin absorbs most strongly at about 450 nm, so a spectral scan of amniotic fluid shows a bump above the baseline there; its height above a drawn baseline is the ΔA450. The result is plotted against gestational age on a chart, such as the Liley graph, to estimate the severity of hemolytic disease of the fetus. Oxyhemoglobin peaks at about 410 to 415 nm, so blood contamination can be recognized and can distort the reading. Doppler of the fetal middle cerebral artery has now largely replaced this test.

   • B: 340 nm is used to follow NADH in enzyme assays.
   • C: 540 nm is used for hemoglobin methods such as cyanmethemoglobin.
   • D: 410 to 415 nm is where oxyhemoglobin absorbs, a sign of blood contamination.

4. Question 4 · Medium

   A ΔA450 result from a pregnancy with maternal anti-D plots in zone III of the Liley graph. This indicates:

   1. AMature fetal lungs, ready for delivery
   2. BNo or only mild fetal hemolysis
   3. CModerate hemolysis needing repeat testing
   4. DSevere fetal hemolysis needing prompt action

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   Answer: D · Severe fetal hemolysis needing prompt action

   The Liley graph plots ΔA450 against gestational age in three zones. Zone I suggests an unaffected or mildly affected fetus, zone II moderate disease that needs repeat testing, and zone III severe hemolysis with a risk of hydrops or fetal death. A zone III result calls for intervention, such as intrauterine transfusion, or delivery if the fetus is mature enough. The Liley graph was designed for the third trimester; Queenan's chart covers earlier gestations.

   • A: ΔA450 measures bilirubin from hemolysis, not lung maturity.
   • B: No or mild hemolysis corresponds to zone I.
   • C: Moderate disease that needs repeat testing is zone II.

5. Question 5 · Easy

   An amniotic fluid lecithin/sphingomyelin (L/S) ratio has traditionally indicated fetal lung maturity when it is at least:

   1. A1.5
   2. B0.5
   3. C2.0
   4. D1.0

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   Answer: C · 2.0

   Sphingomyelin stays fairly constant late in pregnancy, while lecithin, the main surfactant phospholipid, rises in the third trimester. A ratio of 2.0 or more has traditionally indicated mature lungs, and lower ratios carry a higher risk of respiratory distress syndrome. Blood and meconium in the fluid interfere with the result. The test uses slow thin-layer chromatography, and fetal lung maturity testing is now rarely offered.

   • A: 1.5 is below the 2.0 cut-off and is borderline at best.
   • B: 0.5 is far below the maturity cut-off.
   • D: A ratio near 1.0 indicates immature lungs.