Concentration, volume and dilutions

12 ASCP MLS practice questions on Concentration, volume and dilutions, part of Laboratory Operations. The first 12 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 23-year-old woman at 10 weeks' gestation has heavy bleeding and a uterus large for dates, and ultrasound suggests a complete hydatidiform mole. Her serum hCG reads '>1,000,000 IU/L' even with the analyzer's automatic 1:100 dilution; the undiluted measuring range is 1 to 10,000 IU/L. The technologist mixes 20 µL of serum with 380 µL of assay diluent and runs this predilution with the automatic 1:100 dilution still selected. The analyzer reports 62,500 IU/L, with its factor of 100 already applied. What hCG result should be reported?

   1. A1,250,000 IU/L
   2. B125,000,000 IU/L
   3. C1,187,500 IU/L
   4. D62,500 IU/L

   Show answerHide answer

   Answer: A · 1,250,000 IU/L

   The manual predilution is 20 µL of serum in a total volume of 400 µL, a 1:20 dilution. The analyzer has already multiplied its raw reading of 625 IU/L by its own factor of 100 to report 62,500 IU/L, so only the manual factor remains: 62,500 × 20 = 1,250,000 IU/L. The total dilution is 1:2,000, and the raw reading lies within the undiluted measuring range, so the result is valid. Using 19 instead of 20, applying the factor of 100 twice and forgetting the predilution are the usual errors, and the report should state that the result was obtained on a diluted sample.

   • B: 125,000,000 IU/L applies the analyzer's factor of 100 a second time.
   • C: 1,187,500 IU/L uses a factor of 19 (380 ÷ 20), the diluent-to-serum ratio, instead of the total-volume dilution.
   • D: 62,500 IU/L ignores the manual 1:20 predilution.

2. Question 2 · Hard

   A serum sample is diluted 1:20 and assayed, and the diluted result is 3.6 mg/L. The laboratory reports this analyte in mg/dL. What result should be reported?

   1. A720 mg/dL
   2. B7.2 mg/dL
   3. C0.36 mg/dL
   4. D72 mg/dL

   Show answerHide answer

   Answer: B · 7.2 mg/dL

   First correct for the dilution: 3.6 mg/L × 20 = 72 mg/L in the undiluted serum. One liter is 10 dL, so 72 mg/L ÷ 10 = 7.2 mg/dL. Leaving out the unit change gives 72, and multiplying by 10 instead of dividing gives 720. Doing the dilution correction and the unit conversion as two separate, written steps avoids both errors.

   • A: Multiplying by 10 converts in the wrong direction; a deciliter holds less than a liter.
   • C: 0.36 mg/dL converts the units but leaves out the dilution factor.
   • D: 72 is the result in mg/L; the conversion to mg/dL was left out.

3. Question 3 · Medium

   Serum is diluted by adding 0.2 mL to 0.8 mL of saline. Then 0.1 mL of this mixture is added to 0.9 mL of saline. What is the final dilution?

   1. A1:50
   2. B1:15
   3. C1:36
   4. D1:10

   Show answerHide answer

   Answer: A · 1:50

   The first step is 0.2 mL in 1.0 mL total, a 1:5 dilution. The second is 0.1 mL in 1.0 mL total, a 1:10 dilution. Serial dilutions multiply: 5 × 10 = 50, so the final dilution is 1:50. Adding the factors (5 + 10) or multiplying the diluent-to-sample ratios (4 × 9) are common mistakes.

   • B: Adding 5 and 10 is wrong; serial dilution factors multiply.
   • C: 4 × 9 multiplies the diluent-to-sample ratios instead of the dilution factors.
   • D: 1:10 is only the second step and ignores the first dilution.

4. Question 4 · Easy

   A set of twofold serial dilutions starts with 1:2 in the first tube. What is the dilution in the fifth tube?

   1. A1:16
   2. B1:64
   3. C1:32
   4. D1:10

   Show answerHide answer

   Answer: C · 1:32

   Each tube halves the concentration of the one before, so the dilution in tube n is 1:2 to the power n. Tube 5 is 2 × 2 × 2 × 2 × 2 = 32, a 1:32 dilution. Counting one step too few gives 1:16, and one too many gives 1:64. Adding the factors (2 + 2 + 2 + 2 + 2 = 10) is wrong, because factors in a series multiply.

   • A: 1:16 is the dilution in the fourth tube.
   • B: 1:64 is the dilution in the sixth tube.
   • D: Dilution factors in a series multiply; they do not add.

5. Question 5 · Hard

   At each step below, sample (or the previous mixture) is added into the stated volume of diluent. Which two-step preparation gives a final dilution of 1:200?

   1. A20 µL into 380 µL, then 100 µL into 900 µL
   2. B20 µL into 400 µL, then 100 µL into 1,000 µL
   3. C20 µL into 380 µL, then 100 µL into 1,000 µL
   4. D50 µL into 450 µL, then 100 µL into 900 µL

   Show answerHide answer

   Answer: A · 20 µL into 380 µL, then 100 µL into 900 µL

   Each dilution factor is the total volume divided by the sample volume. 20 µL into 380 µL is 20 in 400, a 1:20 dilution, and 100 µL into 900 µL is 100 in 1,000, a 1:10 dilution; together 20 × 10 = 1:200. Adding 400 µL and 1,000 µL of diluent instead gives 1:21 and 1:11, about 1:231 overall. Two 1:10 steps make only 1:100.

   • B: The diluent volumes make the steps 1:21 and 1:11, about 1:231 overall.
   • C: The second step is 1:11, so the total is 1:220.
   • D: Two 1:10 steps multiply to 1:100, not 1:200.

6. Question 6 · Easy

   To prepare 500 µL of a 1:5 dilution of serum in saline, combine:

   1. A125 µL serum and 375 µL saline
   2. B50 µL serum and 450 µL saline
   3. C100 µL serum and 500 µL saline
   4. D100 µL serum and 400 µL saline

   Answer and explanation in LearnFree account

7. Question 7 · Medium

   An amylase result is above the linear range, so 50 µL of serum is mixed with 200 µL of saline and rerun. The analyzer reads 410 U/L. What should be reported?

   1. A410 U/L
   2. B2,050 U/L
   3. C82 U/L
   4. D1,640 U/L

   Answer and explanation in LearnFree account

8. Question 8 · Hard

   A procedure says to dilute samples 1:4 and multiply results by 4. A technologist mixes 1 part sample with 4 parts diluent and applies the factor of 4. How do her reported results compare with the true values?

   1. A20% too low
   2. B20% too high
   3. C25% too high
   4. D25% too low

   Answer and explanation in LearnFree account

9. Question 9 · Hard

   A procedure calls for twofold serial dilutions of undiluted serum, but at every step the technologist transfers 0.5 mL into 1.0 mL of diluent. What dilution is actually in the third tube?

   1. A1:27
   2. B1:81
   3. C1:8
   4. D1:9

   Answer and explanation in LearnFree account

10. Question 10 · Medium

    A bench disinfection procedure calls for 70% ethanol, but only 95% ethanol is in stock. To prepare 250 mL, the technologist uses:

    1. AAbout 184 mL, made up to 250 mL with water
    2. BAbout 66 mL, made up to 250 mL with water
    3. C175 mL, made up to 250 mL with water
    4. DAbout 166 mL, made up to 250 mL with water

    Answer and explanation in LearnFree account

11. Question 11 · Hard

    Tube 1 holds 0.1 mL of serum and 0.9 mL of saline. Four more tubes each hold 0.5 mL of saline, and 0.5 mL is carried from each tube to the next. What is the dilution in tube 5?

    1. A1:80
    2. B1:160
    3. C1:50
    4. D1:320

    Answer and explanation in LearnFree account

12. Question 12 · Medium

    A wash buffer concentrate is labeled 20X. How do you prepare 2 L of working-strength (1X) buffer?

    1. A100 mL concentrate plus 1,900 mL water
    2. B100 mL concentrate plus 2,000 mL water
    3. C50 mL concentrate plus 1,950 mL water
    4. D200 mL concentrate plus 1,800 mL water

    Answer and explanation in LearnFree account