Peripheral blood mononuclear cells proliferation [optical density (OD)] readings exposed to phytohaemagglutinin/lipopolysaccharide (PHA/LPS). All patients from group III compared with only 33% of group I and group II patients had an OD reading above 0·75*. This threshold had a 100% negative predictive value and 40% positive predictive value for intensive care unit (ICU) hospitalization. **P = 0·005 for the difference between ICU versus ward infants; ***P = 0·003 for the difference between ICU versus ambulatory infants. ♦, Group I, ambulatory; •, group II, on ward; ▴, group III, ICU.