Neonatal sepsis can be due to a bacterial, fungal, or viral infection. Early onset sepsis, in the first 7 days, usually results from organisms acquired intrapartum, and symptoms can appear at or soon after birth. The most common organisms include group B streptococcus (GBS) and gram-negative enteric organisms (often E. coli), however organisms may vary according by location. Later onset infections are frequently nosocomial, and often due to foreign bodies (particularly central venous catheters). Preterm infants are more prone to infection and have fatality rates from infection 2-4 times higher than term infants.
Mechanism of Action
Approximately 0.5-1cc of blood is drawn antiseptically from the neonate. Samples are collected directly into a vial containing growth media, typically a liquid broth. Each vial is then incubated at 37C for 2-7 days and observed for growth. Positive cultures are then subcultured on a variety of media known to promote or inhibit the growth of specific organisms. Bacteria can also be Gram stained to help identify broad groups of bacterial organisms to direct antibiotic treatment. Even in high-resource settings, neonatal blood cultures can have low sensitivity because so little blood is drawn. At small blood volumes, only relatively high concentrations of bacteria are detectable. Positive results, therefore, are only evident when the disease has progressed to dangerous levels. For this reason, most caregivers presumptively treat for infection in advance of blood culture completion, and refine treatment based on blood culture results.
Current use in High-Resource Settings
Blood cultures are commonly taken using specialized bottles pre-filled with broth, color changing chemical indicators as well as barcoding used in a specific high volume, automated system. Samples are then loaded into incubators which monitor hundreds of bottles at once, automatically identifying positive cultures. Positive cultures are then manually removed and subcultured on different agar plates to identify the invasive organism. The process requires a microbiology lab and specialized technician. In labs where the number of samples is low, manual systems like the one pictured above may be used instead of the larger, automated systems.
Application in Low-Resource Settings
Given the infrastructure requirements of performing blood cultures, they are less accessible in low-resource settings. Blood cultures may be used to identify the presence of infection, but further testing may not be available to identify a specific organism. Until a rapid, point of care system is available, the majority of neonatal sepsis diagnosis (and especially in the community) will continue to rely on clinical signs. Bang et al. report that 2 of 7 clinical signs “predicted sepsis death sensitivity 100% and specificity 92%. Those seven signs are “reduced or stopped sucking; weak or no cry; limbs becoming limp; vomiting or abdominal distension; baby cold to touch; severe chest indrawing; umbilical infection.” Work is underway to develop clinical guidelines community health workers may be able to use to identify neonates in need of urgent care.