Patients in intensive care units (ICUs) are at significant risk of potentially life-threatening secondary infections, including from antibiotic-resistant bacteria, such as MRSA and C. difficile. Now, a new test could identify those at greatest risk—and speed up the development of new therapies to help at-risk patients.

Infections in ICUs tend to be caused by organisms, such as multiresistant gram-negative bacteria found in the gut, that are resistant to frontline antibiotics. Treating such infections means relying on broad-spectrum antibiotics, which run the risk of breeding further drug resistance, or antibiotics that have toxic side effects.

Estimates of the proportion of patients in the ICU who will develop a secondary infection range from one in three to one in two; around a half of these will be pneumonia. However, some people are more susceptible than others to such infections—evidence suggests that the key may lie in malfunction of the immune system.

In a study published in the journal Intensive Care Medicine, a team of researchers working across four sites in Edinburgh, Sunderland, and London has identified markers on three immune cells that correlate with an increased risk of secondary infection. The team was led by researchers at the Universities of Cambridge and Edinburgh and biotech company BD Bioscience.

“These markers help us create a ‘risk profile’ for an individual,” explains Dr. Andrew Conway Morris from the Department of Medicine at the University of Cambridge. “This tells us who is at greatest risk of developing a secondary infection.”

“In the long-term, this will help us target therapies at those most at risk, but it will be immediately useful in helping identify individuals to take part in clinical trials of new treatments.”

Clinical trials for interventions to prevent secondary infections have met with mixed success, in part because it has been difficult to identify and recruit those patients who are most susceptible, say the researchers. Using this new test should help fine-tune the selection of clinical trial participants and improve the trials’ chances of success.

The markers identified are found on the surface of key immune cells: neutrophils (frontline immune cells that attack invading pathogens), T cells (part of our adaptive immune system that seek and destroy previously encountered pathogens), and monocytes (a type of white blood cell).

The researchers tested the correlation of the presence of these markers with susceptibility to a number of bacterial and fungal infections. An individual who tests positive for all three markers would be at two to three times greater risk of secondary infection compared with someone who tests negative for the markers.

The markers do not indicate which secondary infection an individual might get, but rather that they are more susceptible in general.

“As intensive care specialists, our priority is to prevent patients developing secondary infections and, if they do, to ensure they get the best treatment,” says Professor Tim Walsh from the University of Edinburgh, senior author on the study.