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Amniotic fluid

5 ASCP MLS 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

    A woman with anti-D (titer 128) is at 31 weeks. Her amniotic fluid scan is shown, with the baseline drawn from 365 to 550 nm. At 31 weeks, the laboratory's Liley chart gives zone I below 0.07, zone II from 0.07 to 0.30 and zone III above 0.30. What is the ΔA450, and in which zone does it fall?

    Figure: MedExams
    Semilogarithmic amniotic fluid absorbance scan from 350 to 600 nm with a hump centred at 450 nm. A dashed baseline joins the curve at 365 and 550 nm; at 450 nm the curve reads 0.52 and the baseline 0.18.
    1. AΔA450 0.52, zone III
    2. BΔA450 0.34, zone II
    3. CΔA450 0.18, zone II
    4. DΔA450 0.34, zone III
    Show answer

    Answer: D · ΔA450 0.34, zone III

    ΔA450 is the absorbance of the scan at 450 nm minus the baseline at the same wavelength: 0.52 - 0.18 = 0.34. The baseline removes the background absorbance from turbidity and other pigments, so only the bilirubin hump is measured. A value of 0.34 is above the 0.30 upper limit of zone II at 31 weeks, which places it in zone III and points to severe fetal hemolysis. The raw reading of 0.52 overstates the bilirubin, and 0.18 is only the baseline. The specimen must be protected from light, and blood in it adds an oxyhemoglobin peak near 410 nm that distorts the reading.

    • A: 0.52 is the total absorbance; the baseline was not subtracted.
    • B: 0.34 is above the 0.30 upper limit of zone II at 31 weeks.
    • C: 0.18 is the baseline at 450 nm, not the bilirubin difference.

  2. Question 2 · Medium

    Serial amniotic fluid ΔA450 results from a pregnancy with anti-D are plotted on the Liley-type chart shown. What do they indicate?

    Figure: MedExams
    Semilogarithmic Liley-type chart of delta A450 against gestational age from 27 to 40 weeks, split by two downward-sloping lines into zones I, II and III, with three results: 0.17 at 28 weeks, 0.25 at 30 weeks and 0.40 at 32 weeks.
    1. AWorsening hemolysis, now in zone III
    2. BModerate hemolysis, stable in zone II
    3. CMild hemolysis that is resolving
    4. DThe normal rise as gestation advances
    Show answer

    Answer: A · Worsening hemolysis, now in zone III

    Each result is higher than the one before, and the 32-week value of 0.40 lies above the upper line, in zone III. Zone III means severe hemolytic disease with a risk of fetal hydrops, so intrauterine transfusion or delivery is considered. Bilirubin in normal amniotic fluid falls as pregnancy advances, which is why the zone lines slope downward, so a rising ΔA450 is never a normal finding. Today, Doppler of the fetal middle cerebral artery has largely replaced serial amniocentesis for following these pregnancies.

    • B: The results rise across zone II and the last one is above it, so they are not stable.
    • C: A resolving process would fall toward zone I; these values rise.
    • D: Normal ΔA450 falls with gestational age; it does not rise.

  3. Question 3 · Hard

    Amniotic fluid from a 29-week pregnancy with maternal anti-D is sent for ΔA450 in a clear tube that sat on a sunny bench for 2 hours. Two weeks earlier the ΔA450 plotted in high zone II; today it plots in zone I. What is the most likely explanation?

    1. AMeconium in the fluid falsely lowered the reading
    2. BAmniotic bilirubin normally falls as pregnancy advances
    3. CLight broke down bilirubin, falsely lowering it
    4. DThe fetal hemolysis resolved without treatment
    Show answer

    Answer: C · Light broke down bilirubin, falsely lowering it

    Bilirubin is broken down by light, so amniotic fluid for ΔA450 is protected in an amber or foil-wrapped tube and sent promptly. Here the light exposure most likely lowered the bilirubin and made the hemolysis look milder, which is dangerous in a sensitized pregnancy. The scan from 350 to 550 nm measures the rise in absorbance at 450 nm above a baseline, plotted by gestational age on a Liley graph. A fresh, protected sample should be requested.

    • A: Meconium adds absorbance and tends to raise the reading, not lower it.
    • B: The Liley zones already allow for the normal fall in bilirubin with gestational age.
    • D: Hemolysis from maternal anti-D does not resolve on its own while the antibody persists.

  4. Question 4 · Medium

    In a pregnancy complicated by maternal diabetes, which amniotic fluid finding is the most reliable sign of fetal lung maturity?

    1. AL/S ratio of 2.0
    2. BPhosphatidylglycerol present
    3. CSphingomyelin higher than lecithin
    4. DLamellar body count of 15,000/µL
    Show answer

    Answer: B · Phosphatidylglycerol present

    In diabetic pregnancies the lungs can mature late, and respiratory distress may occur even with an L/S ratio of 2.0. Phosphatidylglycerol (PG) appears late in lung development, so its presence is a stronger sign of maturity in these cases. PG is not found in blood or meconium, so contamination does not cause a false positive. Lecithin and sphingomyelin are about equal until about 32 weeks, and the L/S ratio usually reaches 2.0 by about 35 weeks.

    • A: Diabetic pregnancies can show respiratory distress at an L/S ratio of 2.0.
    • C: Sphingomyelin above lecithin (L/S below 1) indicates immature lungs.
    • D: A count of 15,000/µL indicates immature lungs.

  5. Question 5 · Easy

    Lamellar body counts for fetal lung maturity are performed on:

    1. AA spectrophotometer reading at 450 nm
    2. BA thin-layer chromatography plate
    3. CThe red cell channel of a hematology analyzer
    4. DThe platelet channel of a hematology analyzer
    Show answer

    Answer: D · The platelet channel of a hematology analyzer

    Lamellar bodies are packets of surfactant phospholipid secreted by type II pneumocytes, and they are about the size of platelets. They can therefore be counted in the platelet channel of a standard hematology analyzer. A count of about 50,000/µL or more is generally taken as mature, although cutoffs depend on the analyzer and method. Specimens heavily contaminated with blood or meconium are not suitable.

    • A: 450 nm is used to measure bilirubin in amniotic fluid.
    • B: Thin-layer chromatography is used for the L/S ratio.
    • C: Lamellar bodies are platelet-sized and are counted in the platelet channel.

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