To investigate group differences, the within-group distributions, and the clinical correlates of platelet serotonin (5-HT) levels in pervasive developmental disorders (PDD).

Platelet 5-HT levels were measured in Dutch children and young adults, recruited from 2001 through 2003, with PDD (autism, Asperger’s, and PDD-not otherwise specified [PDD-NOS]; n = 81) or with mental retardation (MR; n = 54) but without PDD, and in normal controls (n = 60). The distribution of platelet 5-HT levels was assessed using mixture-modeling analyses. Relationships between platelet 5-HT levels and a full range of demographic, clinical, and behavioral variables were examined.

Group mean (± SD) platelet 5-HT levels (nmol/10⁹ platelets) were significantly higher in the autistic (4.51 ± 1.61, n = 33) and PDD-NOS (4.90 ± 1.54, n = 43) groups compared to the MR (3.48 ± 1.33, n = 54) or the normal control (3.58 ± 1.08, n = 60) groups (F_4,190 = 9.35, p < .001). Platelet 5-HT values in the combined PDD group showed a bimodal distribution, and an empirical cutpoint for hyperserotonemia was determined. None of the behavioral variables examined was significantly associated with platelet 5-HT levels.

The platelet hyperserotonemia of autism was replicated in Dutch subjects. Platelet 5-HT levels were also increased in PDD-NOS, while no elevation was seen in MR. Platelet 5-HT levels appeared to be bimodally distributed in the PDD group, with an apparent hyperserotonemic subgroup.