Question 1

How do you correct calcium for albumin?

Accepted Answer

`Corrected Ca = measured Ca + 0.8 × (4.0 − albumin)`. About 40% of serum calcium is bound to albumin, so total calcium underestimates physiologically active (ionized) calcium in hypoalbuminemia. When a clinical decision hinges on the result, get a **direct ionized calcium** — the correction is an estimate and breaks down at extremes of albumin or pH.

Question 2

How does hyperglycemia affect serum sodium?

Accepted Answer

Hyperglycemia pulls water into the vascular space, diluting sodium (pseudohyponatremia). Use the **Katz correction** (sodium drops ~1.6 mEq/L per 100 mg/dL of glucose above 100) for glucose <400, and the **Hillier correction** (~2.4 mEq/L per 100 mg/dL) for glucose ≥400, since the relationship is nonlinear at extreme hyperglycemia.

Question 3

What is the reticulocyte production index (RPI)?

Accepted Answer

RPI corrects the reticulocyte percentage for both anemia severity and reticulocyte maturation time:

1. Corrected retic = retic % × (Hct / 45)
2. Divide by maturation factor (1.0 / 1.5 / 2.0 / 2.5 based on Hct)

**RPI ≥ 2** suggests adequate marrow response (hemolysis or blood loss). **RPI < 2** suggests underproduction (iron deficiency, anemia of chronic disease, marrow failure).

Question 4

Why is raw reticulocyte percentage misleading in anemia?

Accepted Answer

Reticulocytes are reported as a percentage of total RBCs. In anemia, the denominator drops, so a percentage can look adequate even when absolute reticulocyte production is low. The RPI corrects for both this dilution effect and the prolonged maturation of reticulocytes released early from the marrow.

Corrected Labs

Corrected Calcium

Corrected Sodium

Reticulocyte Production Index

References

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