Special tests: titrations

5 ASCP MLS practice questions on Special tests: titrations, part of Blood Banking. 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 · Medium

   In pregnancy, a maternal antibody titer is mainly used to:

   1. AIdentify the antibody's specificity
   2. BPredict the father's red cell phenotype
   3. CDecide when the fetus needs closer monitoring
   4. DCalculate the dose of Rh immune globulin to give

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   Answer: C · Decide when the fetus needs closer monitoring

   Titration estimates how much antibody is present by testing serial dilutions of plasma. When the titer reaches the laboratory's critical level, the fetus is monitored with middle cerebral artery Doppler. A titer alone does not predict how severely the fetus is affected. Specificity is found by antibody identification, not titration.

   • A: Specificity comes from antibody identification with a panel.
   • B: The father's phenotype is found by typing his red cells.
   • D: The Rh immune globulin dose depends on the size of a fetal bleed.

2. Question 2 · Hard

   Two samples from a pregnant woman, tested in parallel, both have a titer of 16, but the titration score rises from 38 to 52. What does the score add?

   1. AA switch of the antibody from IgG to IgM
   2. BA change in the antibody's specificity
   3. CA misread titer that must be repeated with new cells
   4. DStronger reactions overall, despite the same titer

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   Answer: D · Stronger reactions overall, despite the same titer

   A titration score gives each reaction grade a value and adds them across all dilutions. It can show a rise in antibody activity even when the endpoint dilution stays the same. A score difference of about 10 or more is usually considered significant. The score is read together with the titer and the clinical picture.

   • A: Scores do not show antibody class.
   • B: Titration measures amount, not specificity.
   • C: A different score with the same titer is expected, not a sign of misreading.

3. Question 3 · Hard

   Doubling dilutions of plasma give: 1:1 4+, 1:2 3+, 1:4 2+, 1:8 1+, 1:16 w+, 1:32 0. If the endpoint is the highest dilution with a 1+ reaction, what is the titer?

   1. A32
   2. B4
   3. C8
   4. D16

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   Answer: C · 8

   The titer is the reciprocal of the highest dilution meeting the endpoint, here the 1:8 dilution with 1+ agglutination. The weak (w+) reaction at 1:16 does not meet the 1+ endpoint. Using the same endpoint, method and cells every time is what makes titers comparable. Reactions are read at the antiglobulin phase for IgG antibodies.

   • A: 32 is the first negative dilution.
   • B: 4 is the last 2+ dilution, not the 1+ endpoint.
   • D: 16 uses the weak reaction, which is below the 1+ endpoint.

4. Question 4 · Hard

   A woman at 22 weeks of pregnancy has anti-c. Her frozen 16-week sample is titrated in parallel with today's sample, using the same R2R2 cells and a saline antiglobulin method. The laboratory reports the titer as the reciprocal of the highest dilution giving a 1+ reaction, and its critical titer is 16. 16-week sample: 1:1 3+, 1:2 3+, 1:4 2+, 1:8 1+, 1:16 w+, 1:32 0. Today: 1:1 4+, 1:2 3+, 1:4 3+, 1:8 2+, 1:16 2+, 1:32 1+, 1:64 w+, 1:128 0. The correct interpretation is:

   1. ATiter 32: fourfold rise, now critical
   2. BTiter 64: fourfold rise, now critical
   3. CTiter 32: within normal test variation
   4. DTiter 32: twofold rise, not significant

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   Answer: A · Titer 32: fourfold rise, now critical

   With the 1+ endpoint, the 16-week titer is 8 and today's titer is 32. Because both samples were tested together with the same cells and method, this two-tube (fourfold) rise is significant; only a one-tube difference is within normal test variation. A titer of 32 is above the laboratory's critical titer of 16, so the fetus is now at risk and is usually followed by middle cerebral artery Doppler ultrasound. Reading the w+ tubes as endpoints gives 16 and 64, which this laboratory does not use, and mixing the two readings gives a false twofold change.

   • B: Reading the w+ tubes as the endpoint gives 64, but this laboratory uses a 1+ endpoint.
   • C: Only a one-tube difference is within test variation; this rise is two tubes.
   • D: The 16-week titer is 8, not 16, so the rise is fourfold.

5. Question 5 · Medium

   The table shows a prenatal anti-D titration with its score key. The titer and score are:

   

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   1. ATiter 64, score 19
   2. BTiter 32, score 60
   3. CTiter 128, score 65
   4. DTiter 64, score 65

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   Answer: D · Titer 64, score 65

   The titer is the reciprocal of the highest dilution giving a 1+ reaction, here 1:64, so the titer is 64. The score adds the value of every tube: 12 + 12 + 10 + 10 + 8 + 8 + 5 = 65. Samples are compared by testing them in parallel; a rise of more than one dilution, or a score change of 10 or more, is significant. For anti-D, a titer at or above the laboratory's critical value (often 16) leads to fetal monitoring.

   • A: 19 adds the grades (4 + 4 + 3 and so on) instead of the score values.
   • B: 1:32 is the last 2+ tube; the endpoint is the last 1+ tube, and 60 leaves out its score.
   • C: 1:128 is the first nonreactive tube, not the endpoint.