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The percent transferrin saturation (TSAT) and serum ferritin (SF) are indirect measures
of iron status and are at present the two most commonly used tests to diagnose absolute
and functional iron deficiency in patients with chronic kidney disease.
Absolute iron deficiency may be diagnosed when TSAT is <20% and SF is <100 ng/mL
for patients with chronic kidney disease.[5]
Functional iron deficiency may be more difficult to diagnose since iron status parameters
may indicate adequate iron stores. Patients on epoetin with functional iron deficiency
have TSAT <20% and normal serum ferritin levels (≥100 ng/mL) but experience
hemoglobin or hematocrit decreases at the same or an increased epoetin dose. Patients
with functional iron deficiency may experience an increase in hemoglobin/hematocrit
after a course of IV iron therapy.
Functional iron deficiency also must be differentiated from inflammatory iron block,
which can also occur in patients with chronic inflammatory conditions such as infections,
certain malignancies, and autoimmune disorders. In this case, both hemoglobin and
hematocrit are reduced, and TSAT may be <20%.[5]
SF levels, on the other hand, may be 100 to 700 ng/mL or higher. In patients with
functional iron deficiency, serial levels of SF decrease during epoetin alfa therapy,
yet remain elevated (≥100 ng/mL); in contrast, patients with inflammatory iron
block usually show an abrupt increase in SF along with a sudden drop in TSAT.[5]
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