Neonatal sepsis remains a burden problem by showing minimal initial symptoms of subtle character, nonspecific manifestation, and diagnostic pitfalls. The clinical course can be fulminant and fatal if treatment is not commenced promptly. It is therefore crucial to establish early diagnosis and initiate adequate therapy. Besides clinical symptoms, the most reliable laboratory markers in establishing diagnosis is currently the combined measurement of CRP and a cytokine (IL-6 and IL-8). Due to their different kinetics, a diagnostic gap might occur and thus withholding antimicrobial therapy in clinical suspicion of infection is not acceptable. We therefore need parameters which unerringly differentiate between infants in need for antimicrobial therapy and those who are not. Flow cytometry promises to be a useful tool in this field, allowing the determination of different cellular, dissolved, and functional pathophysiological components of sepsis. Despite technical and methodical advances in flow cytometry, its use in clinical routine is still limited. Advantages and disadvantages of promising new parameters in diagnosis of sepsis performed by flow cytometry, particularly CD64, HLA-DR, and apoptosis, are reviewed here. The necessity of tests to be used as an “ideal” parameter is presented. 1. Introduction Sepsis in the newborn is a common disorder affecting 1.1 to 2.7% of all newborns [1]. It is classified into early-onset form (EONS) within the first 72 hours of life and late-onset form (LONS) afterwards. Prematurity predisposes to sepsis: premature infants with a birth weight less than 1000？g (ELBW: extremely low birth weight infants) are particularly at risk with an inverse correlation between gestational age, birth weight, and sepsis [1]. Even late-preterm infants (LPI) have a fourfold higher risk of sepsis than term infants [2]. Thus, bacterial infections remain the most common cause for mortality and morbidity in early human life. Clinical symptoms are variable, minimal, and nonspecific [1, 3]. Deficiencies of both innate and adaptive immunity contribute to the impaired neonatal host defence (reviewed in [4]). A domination of na？ve immune cells, functional impairments [5], and lower leukocyte subset numbers contribute further to an increased susceptibility [6], although basic functions, such as recognition and phagocytosis of bacteria, are already developed in the same proportion as in adults [7, 8]. In response to bacteremia, a systemic inflammation response syndrome (SIRS) occurs in preterm infants with rapid secretion mainly of proinflammatory
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