Predicting Sepsis

A team of Harvard Medical School investigators at Massachusetts General Hospital has identified what may be a biomarker predicting the development of sepsis, a dangerous systemic infection in patients with serious burns.

When white blood cells called neutrophils pass through a microfluidic device, their motion is significantly altered two to three days before sepsis occurs, which may provide a critically needed method for early diagnosis. The researchers report their findings in the open-access journal PLOS ONE.

“Neutrophils are the major white blood cell protecting us against infection, and a healthy individual has an army of 25 billion circulating neutrophils ready to fight invading pathogens,” explained Daniel Irimia, HMS assistant professor of surgery at Mass General and corresponding author of the PLOS ONE report. “The most common blood test ordered to evaluate a patient’s ability to fight infection is absolute neutrophil count, based on the assumption that, like well-trained soldiers, neutrophils are always fast, disciplined and effective in pursuing their targets, meaning that the size of the neutrophil ‘army’ is all that matters. Our work challenges that assumption and shows that, even when the number of neutrophils is unchanged, the army can fall into disarray and become ineffective.”

Sepsis is the leading cause of death among patients with major burns. Among patients whose burns affect more than 20 percent of body surface, the mortality rate is 30 percent. Every 6 hours of delay in a sepsis diagnosis decreases the chances of survival by 10 percent.

Because the symptoms of sepsis are similar to those of the systemic inflammation that occurs in almost every serious burn patient, diagnosing sepsis relies on culturing bacteria from the blood, a process that takes 12 to 24 hours.

No previous studies have identified sepsis-associated changes in the motion of neutrophils, which travel to sites of infection in response to chemical signals. But neutrophils’ ability to move spontaneously is known to be less efficient in burn patients than in healthy individuals. This led the researchers to wonder whether changes in neutrophil motility might be correlated with sepsis in patients with major burns.

To investigate this possibility, the research team designed a microfluidic device with channels smaller than the diameter of neutrophils to study the cells’ motion toward a chemical signal. Straight channel sections measured the speed and persistence of the cells’ motion; divisions and obstacles in the channels tested the cells’ ability to change directions.

The researchers then analyzed the ability of neutrophils from blood samples of 13 patients with serious burns, collected several times during their treatment, to move through the device when it was primed with one of two chemical attractants or with saline solution. They then compared movement of those cells with the movement of cells from 3 healthy volunteers.

Neutrophils from healthy individuals moved quickly and efficiently through the device toward a chemical attractant, easily navigating around corners and posts, but cells from burn patients showed limited, slower and poorly organized movement toward the chemical signal.

Analyzing movement patterns when the device contained no chemical attractant revealed a surprising finding: Neutrophils from patients who had developed or were about to develop sepsis spontaneously moved through the device-like soldiers deciding to advance in the absence of any orders-while those from other patients and from healthy volunteers showed little or no motion.

This movement of neutrophils in the absence of chemical signals was observed in samples taken from some patients several days before a diagnosis of sepsis could be made. Once effective antibiotic treatment began, the unusual movement pattern began to fade.

In addition to allowing faster initiation of antibiotic treatment, the ability to diagnose sepsis rapidly and accurately could reduce the inappropriate use of antibiotics. The proliferation of resistant bacteria is common in burn units, the authors noted.

“Since only a handful of rare genetic disorders affect neutrophil function, it has long been assumed that studying these cells was not important; but our findings indicate that neutrophils play a much more important role in sepsis than has been appreciated,” said Irimia, who is associate director of the BioMEMS Resource Center in the Mass General Department of Surgery. “Studies including larger numbers of patients with major burns and more precise measurements are under way. We’re also working to expand this investigation to other patients at risk for sepsis, to see if the findings from burn patients have broader application.”