Required section · Section 5 of 6

Classify, calculate, and select the next test

Cases B–D use the same timing assumption: CBC, reticulocytes, and smear are from one stable pretreatment specimen unless stated otherwise. The adult intervals are hemoglobin 12.0–16.0 g/dL, hematocrit 36.0–46.0%, MCV 80–100 fL, and ARC 25–100 × 10^9/L. Calculate ARC from the numeric RBC count and reticulocyte percentage before choosing one next test.

Case B: after a febrile illness 2 days earlier, hemoglobin 8.9 g/dL, hematocrit 27%, RBC 3.00 × 10^12/L, reticulocytes 8.0%, MCV 90 fL, and smear with spherocytes and polychromasia. ARC = 3.00 × 0.080 = 240 × 10^9/L; corrected percentage = 8.0 × 27 ÷ 45 = 4.80%; RPI = 4.80 ÷ 2.0 = 2.40. The response is increased for anemia; one defensible next test is a direct antiglobulin test. Case C: after 3 months of fatigue, hemoglobin 11.2 g/dL, hematocrit 34%, RBC 3.20 × 10^12/L, reticulocytes 1.0%, MCV 106 fL, and macro-ovalocytes with hypersegmented neutrophils. ARC = 3.20 × 0.010 = 32 × 10^9/L; corrected percentage = 1.0 × 34 ÷ 45 = 0.76%; RPI = 0.76 ÷ 2.0 = 0.38. The response is inadequate; one defensible next test is serum vitamin B12.

Case D: six weeks after an antihypertensive therapy change, hemoglobin 9.4 g/dL, hematocrit 26.7%, RBC 3.00 × 10^12/L, reticulocytes 1.0%, MCV 89 fL, and no specific morphology on the declared review. ARC = 3.00 × 0.010 = 30 × 10^9/L; corrected percentage = 1.0 × 26.7 ÷ 45 = 0.59%; RPI = 0.59 ÷ 2.0 = 0.30. The response is inadequate; one defensible next test is serum creatinine. Novel calculation case E: from a stable pretreatment specimen drawn one day after presentation, hemoglobin 7.8 g/dL, hematocrit 26%, RBC 2.80 × 10^12/L, reticulocytes 4.0%, MCV 93 fL, and polychromasia with no schistocytes identified in 1,000 reviewed RBCs. ARC = 2.80 × 0.040 = 0.112 × 10^12/L = 112 × 10^9/L; corrected percentage = 4.0 × 26 ÷ 45 = 2.31%; RPI = 2.31 ÷ 2.0 = 1.16. ARC 112 × 10^9/L is a meaningfully increased response for hemoglobin 7.8 g/dL, not merely a value that clears the reference interval; because objective 1 makes ARC the primary measure, this elevated marrow output governs over the instructional RPI 1.16, which reads as limited only because the maturation-factor convention does not fully capture output relative to this degree of anemia. One defensible next test is haptoglobin, LDH, and unconjugated bilirubin to evaluate the elevated response for a hemolytic contribution.

Cases B–E: numeric response, timing, morphology, calculation, and one endpoint. Adult intervals: hemoglobin 12.0–16.0 g/dL, hematocrit 36.0–46.0%, MCV 80–100 fL, ARC 25–100 × 10^9/L.
Case	Hemoglobin / hematocrit / context	MCV / RBC / reticulocytes / timing	Visible morphology	Classification calculation	One next test
B	8.9 g/dL / 27% / febrile illness	90 fL / 3.00 × 10^12/L / 8.0% / 2 days	Spherocytes, polychromasia	ARC 240 × 10^9/L; corrected 4.80%; RPI 2.40; increased	Direct antiglobulin test
C	11.2 g/dL / 34% / fatigue	106 fL / 3.20 × 10^12/L / 1.0% / 3 months	Macro-ovalocytes, hypersegmented neutrophils	ARC 32 × 10^9/L; corrected 0.76%; RPI 0.38; inadequate	Serum vitamin B12
D	9.4 g/dL / 26.7% / therapy change	89 fL / 3.00 × 10^12/L / 1.0% / 6 weeks	No specific morphology on declared review	ARC 30 × 10^9/L; corrected 0.59%; RPI 0.30; inadequate	Serum creatinine
E	7.8 g/dL / 26% / stable pretreatment specimen	93 fL / 2.80 × 10^12/L / 4.0% / 1 day	Polychromasia; no schistocytes in 1,000 reviewed RBCs	ARC 112 × 10^9/L; corrected 2.31%; RPI 1.16; adequate (ARC governs; increased response for Hgb 7.8)	Haptoglobin, LDH, bilirubin

Ordering exercise

Put the case-review actions in the order that prevents premature etiologic labeling.

1. Select focused follow-up

Choose the domain that best separates the remaining mechanisms.
2. Review RDW and smear

Expose dimorphism and morphology evidence.
3. Check context and specimen

Verify acceptability, local interval, and transfusion or therapy timing.
4. Classify on the two-axis grid

Use MCV and marrow response.

Knowledge checks

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Knowledge check 1

For novel case E, which calculation and next test correctly integrate the stable pretreatment data?

Choose one option.

ARC 112 × 10^9/L is a meaningfully increased response for hemoglobin 7.8 g/dL, not merely above the interval; ARC governs as the primary measure, so the response is adequate; obtain haptoglobin, LDH, and bilirubin to evaluate the elevated response.

ARC 112 × 10^9/L; hemolysis established; haptoglobin.

No calculation is possible because a reticulocyte percentage cannot be used.

Knowledge check 2

Which combined findings support hemolysis follow-up rather than relying on reticulocytosis alone? Select all that apply.

Choose at least 3 options.

Low haptoglobin

Increased lactate dehydrogenase

Increased unconjugated bilirubin

Reticulocytosis alone without other markers

Section status

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