Additionally, peak systolic velocity may be used, in which case a measurement of 250 cm/s is used as a threshold for prophylactic transfusion.556 In general, younger children and those with relatively high cerebral blood flow velocities should be monitored more frequently because of a higher risk of conversion to abnormal velocities in younger patients and in those with TCD velocities closer to 200 cm/s.557 Despite strong evidence of its value, overall TCD screening rates continue to be suboptimal as a result of patient and pro- The National Institutes of Health and the American Academy of Pediatrics recommend annual TCD screening from 2 to 16 years of age.560,561 Few studies have been done to determine whether TCD also predicts stroke in adults with SCD. One study comparing TCD velocities in SCD adults with those of healthy control subjects found that velocities in SCD adults were lower than those found in children, higher than in healthy control sub- jects, and negatively correlated with the hematocrit in both Another study found no examples of high TCD (>200 cm/s) in adults with SCD. The mean velocity was 110 cm/s, which is higher than in normal adults but lower than in children with SCD.563 At present, there are no validated TCD criteria for predicting stroke in adults with SCD. AlthoughTCD remains the most extensively validated stroke prediction tool, other clinical characteristics are also associated with increased risk of stroke. One study found that nocturnal desaturation predicted neurological events in 95 patients with SCD (median age, 7.7 years; range, 1 to 23 years) followed up for a median of 6 years.564 There were 7 strokes among 19 individuals with events. 