In neonatal intensive care, a tiny vial of blood can carry an enormous clinical decision. When a newborn is suspected of having sepsis, clinicians often begin antibiotics immediately, long before laboratory confirmation is available. A blood-culture instrument may later signal that microorganisms are growing, and the time required to produce that signal—known as time to positivity, or TTP—can influence whether treatment is continued, narrowed or stopped. A new article by H.N. Rodriguez and R.F. Hamdy, published in the Journal of Perinatology, draws attention to the caution required when interpreting this clock. The central message is that TTP is valuable, but it is not a simple countdown that can independently rule infection in or out.
Neonatal sepsis is particularly difficult to diagnose because its earliest signs are often subtle and nonspecific. Temperature instability, breathing difficulties, feeding intolerance, lethargy, changes in blood pressure and abnormal laboratory values may be caused by infection, prematurity, birth-related stress or other medical complications. Blood culture remains the standard laboratory method for identifying bacteria or fungi circulating in the bloodstream, but the test is constrained by the small amount of blood that can safely be collected from a newborn. That limitation matters because the probability of detecting an organism depends partly on how many microbial cells are present in the sample. A negative result after a certain period therefore does not carry the same meaning in every infant or every clinical situation.
TTP measures the interval between a culture bottle entering an automated monitoring system and the moment the instrument detects metabolic activity consistent with microbial growth. Modern systems continuously monitor changes such as carbon dioxide production or other chemical signals generated as organisms multiply. In general, cultures containing a larger inoculum may become positive sooner, while bottles with very few organisms may take longer. Some rapidly growing bacteria can trigger an alert within a relatively short time, whereas slower-growing organisms may require substantially longer incubation. The result is a dynamic biological measurement, not a direct measure of disease severity. The clock begins only after the sample has been collected and processed, and it cannot compensate for an inadequate specimen or antibiotics given before collection.
One of the most important variables is the volume of blood inoculated into the culture bottle. In neonates, clinicians must balance diagnostic yield against the risk of iatrogenic blood loss, particularly in extremely premature infants who may undergo frequent laboratory testing. A small sample can contain no organisms even when infection is present, simply because bacteria or fungi were not captured in the aliquot. If the sample does contain an organism, the low number of cells may also delay the positivity signal. This means that a prolonged TTP can reflect a low microbial burden or a limited specimen rather than the absence of clinically meaningful infection. Conversely, a very early positive result may indicate a higher concentration of organisms, but it still requires clinical interpretation and organism identification.
The timing of antibiotic administration adds another layer of complexity. When antimicrobial therapy begins before blood is drawn, susceptible organisms may be damaged or suppressed, reducing the likelihood that the culture will become positive. Even when a bottle eventually signals growth, prior treatment may alter the apparent kinetics. The interval between collection and incubation can also influence recorded TTP, as can delays in transporting the specimen from the bedside to the laboratory. Accurate interpretation therefore depends on knowing the complete sequence of events: when the blood was collected, when antibiotics were administered, when the bottle entered the instrument and whether any processing delay occurred. A number displayed in a laboratory report may conceal these clinically important intervals.
The identity of the organism is equally important. A culture that grows a recognized pathogen, such as a member of the Enterobacterales, Staphylococcus aureus or group B Streptococcus, carries a different implication from one that grows a skin-associated organism commonly introduced during collection. Coagulase-negative staphylococci, for example, can cause genuine bloodstream infection in vulnerable premature infants, especially those with central venous catheters, but they are also frequent contaminants. TTP may contribute to the assessment: contamination can sometimes be associated with delayed positivity or growth in only one of several bottles. Yet no single timing threshold can reliably distinguish contamination from infection in every newborn. The number of positive cultures, the organism recovered, the presence of an indwelling device and the infant’s clinical course must be considered together.
The meaning of a negative culture is also conditional rather than absolute. Clinicians often use predefined observation periods to decide whether empiric antibiotics can be discontinued when an infant remains clinically stable and cultures show no growth. Such decisions can reduce unnecessary antimicrobial exposure, which is important because prolonged antibiotic treatment may disrupt the developing microbiome, increase drug-related toxicity and contribute to antimicrobial resistance. However, a negative result is reassuring only within the context of the specimen’s quality, the infant’s symptoms, the timing of collection and the antibiotics already given. Persistent clinical deterioration, abnormal physiology or a strong risk profile may justify further evaluation even when the initial culture remains negative.
The article’s emphasis has implications beyond the laboratory report. It encourages neonatal teams to treat TTP as one component of a larger diagnostic framework rather than as an automatic stop-or-continue rule. Electronic health records and automated culture systems can make a time interval appear precise, but precision is not the same as certainty. A value recorded to the minute may still be difficult to interpret if blood volume is unknown, collection was delayed, multiple bottles were not obtained or antimicrobial therapy preceded sampling. Better documentation of these factors could improve the usefulness of TTP in research and clinical practice. Future studies may also help define how timing behaves across gestational ages, birth weights, organisms, culture systems and patterns of antibiotic exposure.
For parents and clinicians, the practical lesson is that neonatal blood-culture timing provides evidence, not a verdict. A rapidly positive culture demands urgent attention, but a delayed signal must be interpreted alongside the organism and the infant’s condition. A culture that remains negative can support stopping treatment in the right circumstances, yet it cannot erase a compelling clinical picture of infection. Rodriguez and Hamdy’s discussion underscores why neonatal sepsis decisions require laboratory science, bedside observation and careful knowledge of how a specimen was obtained. In newborn medicine, where the available blood may be measured in millilitres and the consequences of both missed infection and unnecessary treatment are substantial, understanding what the culture clock can—and cannot—say is essential.
Subject of Research: Interpretation of neonatal blood-culture time-to-positivity in the assessment of suspected neonatal sepsis
Article Title: Important considerations in interpreting neonatal blood culture time-to-positivity
Article References: Rodriguez, H.N., Hamdy, R.F. “Important considerations in interpreting neonatal blood culture time-to-positivity.” Journal of Perinatology (2026). https://doi.org/10.1038/s41372-026-02872-x
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41372-026-02872-x
Keywords: neonatal sepsis, blood culture, time-to-positivity, bloodstream infection, premature infants, antimicrobial therapy, microbiology, diagnostic interpretation
