Oliveira MC, Fernandes RA, Rodrigues CL, Ribeiro DA, Giovanardi MF, Viana MB blood count, blood smear, reticulocyte count, bilirubin concentration, positive osmotic fragility test and negative direct antiglobulin (Coombs') test. Red cell osmotic fragility was evaluated in tubes containing red blood cells washed in solutions of increasing higher sodium chloride (NaCl) concentrations with and without incubation for 24 hours.(3-9) The eosin-5'-maleimide flow cytometry test is not available in our laboratory.(10) Family studies were carried out for all patients. Follow-ups were scheduled at three-month intervals in our outpatient clinic or more frequently if considered necessary. Folic acid was prescribed to all patients. The clinical presentation at baseline was retrospectively classified as mild, moderate or severe based on the modified criteria of Eber et al.(11) using hematological parameters – hemoglobin (Hb) and bilirubin concentrations, and reticulocyte count. The patients were annually screened for cholelithiasis by ultrasonography starting at the age of 7 years old or at any age if the patient had symptoms of cholelithiasis. Patients submitted to splenectomy received immunization for Haemophilus influenzae and Streptococcus pneumoniae 15 days before the procedure. Oral or intramuscular penicillin as prophylaxis was recommended up to the age of 18 years old even though some authors recommend lifelong prophylaxis.(12) The events of interest were transfusions, cholelithiasis diagnosed by the detection of gallstones at ultrasonography, spleen sequestration defined as an increase in the size of the spleen associated with a decrease in Hb and increased reticulocyte count, splenectomy, and aplastic crisis characterized by decreases in Hb concentration and reticulocyte count. Statistical analysis Data analyses were performed using SPSS version 11.5. The time to cholelithiasis was evaluated from the date the patient first frequented the hematology service until the date of ultrasound diagnosis. Differences between subgroups were compared by the two-sided log rank test. The cumulative proportion of events over time was estimated by one minus survival curve of Kaplan Meier. Differences in distributions of dichotomous variables were analyzed by chi-square or Fisher's exact test. The Kruskal-Wallis nonparametric test was used to compare values of non- Gaussian variables. The study was approved by the Research Ethics Committee of UFMG. Results Baseline clinical and demographic characteristics of patients with hereditary spherocytosis according to severity classification are shown in Table 1. The study group comprised 35 males (55.6%) and 28 females (44.4%) belonging to 52 unrelated families. The median age at diagnosis was 5.2 years (range: 5 months to 21 10 Rev Bras Hematol Hemoter. 2012;34(1):9-13