Abstract: A rapid assay for infection in immunodeficient patients such as neonates or immunocompromised patients (e.g. HIV or transplant patients) allows diagnosis at initial evaluation, such that antibiotic treatment and confinement to an intensive care unit can be avoided for uninfected patients. The assay can be used for detecting bacterial, viral, or fungal colonization of the blood stream, cerebrospinal fluid (CSF), or urinary tract. The method is particularly useful for sepsis diagnosis. Polymorphonuclear leukocyte (PMN, neutrophil) CD11b (Mac-1, CR3) levels are measured by flow cytometry or laser scanning microscopy in low volume (0.1 ml) whole blood samples. A dual-laser FACS identifies neutrophils by FITC-conjugated anti-CD15 fluorescent antibodies, and identifies surface neutrophil CD11b marked with PE-conjugated anti-CD11b antibodies. Spontaneous upregulation of CD11b is prevented by handling samples at 4.degree. C. or adding a stabilizing compound such as anti-CD14 antibody or adenosine to the samples.