For every hospital admission, we defined the mean, maximum, minimum, and most recent hematocrit values in the 14 days before the first administration of rHuEPO therapy. To assess iron stores, we recorded the maximum, minimum, mean, and most recent values for serum iron, total iron binding capacity, and ferritin in the 90 days before the administration of rHuEPO therapy. Transferrin saturation (TSAT) was calculated from the highest serum iron value divided by the lowest iron binding capacity in the 90 days before drug administration. We divided iron status into 3 categories: absolute iron deficiency (serum ferritin concentration of <100 ng/mL), indeterminate iron stores (TSAT <20% and serum ferritin concentration of >100 ng/mL), and adequate or replete iron stores (TSAT >20% and serum ferritin concentration of >100 ng/mL). For patients with CKD, we defined indeterminate iron stores as functional iron deficiency. For patients without CKD, this category is consistent with anemia of chronic disease.24 While not all patients received outpatient care at the same facility, we assumed that patients with results documented for commonly performed outpatient laboratory tests received local ambulatory care. To study this, we obtained results for 13 additional laboratory tests (the levels of serum creatinine, rHuEPO, glycosolated hemoglobin, potassium, vitamin B12, folic acid, serum urea nitrogen, thyrotropin, and alanine aminotransferase; prostate-specific antigen concentration; platelet and white blood cell counts; and mean corpuscular volume) from the 90 days before the admission. All laboratory test results used for this analysis were available to the rHuEPO prescriber at the time of the initial drug order through the hospital electronic laboratory reporting system.