BUN/K Ratio Trajectories and MAKE-30 Risk in Subarachnoid Hemorrhage Patients

A new study published in Nature reveals that patterns of change in the blood urea nitrogen-to-potassium (BUN/K) ratio over time can help predict the risk of major adverse kidney events within 30 days among critically ill patients with non-traumatic subarachnoid hemorrhage. Researchers analyzed data from the MIMIC-IV database to identify distinct trajectories of the BUN/K ratio and found that certain patterns were strongly associated with increased risk of acute kidney injury, dialysis, or death.

The study, led by investigators from multiple institutions, focused on patients admitted to intensive care units following a non-traumatic subarachnoid hemorrhage, a type of bleeding in the space around the brain often caused by a ruptured aneurysm. These patients are at high risk for kidney complications due to factors such as reduced blood flow, exposure to contrast agents, and the physiological stress of neurological injury. Early identification of those likely to develop kidney problems could allow for timely interventions to prevent worsening outcomes.

Using a statistical approach known as latent class growth modeling, researchers grouped patients based on how their BUN/K ratio changed during the first few days of ICU stay. Three distinct trajectories emerged: a low-stable group, a moderate-increasing group, and a high-increasing group. Patients in the high-increasing trajectory showed significantly higher rates of MAKE-30, defined as a composite of acute kidney injury requiring dialysis, persistent renal dysfunction, or death from any cause within 30 days of admission.

The BUN/K ratio is a simple, routinely available laboratory marker that reflects both kidney function and volume status. Urea nitrogen levels rise when the kidneys are less able to filter waste, while potassium can fluctuate due to cellular shifts, medication effects, or renal handling. While neither marker alone is specific, their ratio may offer a more nuanced signal of evolving physiological stress in critically ill patients.

Previous research has linked abnormalities in BUN and potassium levels individually to poor outcomes in various critical illnesses, but few studies have examined their combined trajectory over time. This study suggests that tracking how the BUN/K ratio evolves may provide additive value beyond single measurements, potentially helping clinicians stratify risk and guide monitoring or treatment decisions in a vulnerable population.

The analysis included over 500 adult patients with non-traumatic subarachnoid hemorrhage admitted to ICUs represented in the MIMIC-IV database, a large, publicly available dataset of de-identified health information from Beth Israel Deaconess Medical Center in Boston. Researchers controlled for factors such as age, sex, baseline kidney function, severity of neurological injury, and use of medications that could affect electrolyte levels. Even after adjustment, the association between unfavorable BUN/K trajectories and MAKE-30 remained statistically significant.

Experts not involved in the study noted that while the findings are promising, the BUN/K ratio is influenced by many factors beyond kidney function, including gastrointestinal bleeding, steroid use, and muscle breakdown, which could limit its specificity. They emphasized that the ratio should be interpreted in context and not used in isolation to guide clinical decisions.

The researchers acknowledged that the study was observational and cannot prove that changes in the BUN/K ratio directly cause worse kidney outcomes. Instead, the ratio may serve as a marker of underlying physiological strain. They called for future research to validate the findings in other populations and to explore whether interventions targeting fluid balance, nephrotoxin avoidance, or early renal support could modify the risk associated with adverse BUN/K trajectories.

Despite these limitations, the study highlights the potential of routinely collected laboratory data to uncover meaningful patterns in critical illness. As electronic health records grow richer and analytical tools more sophisticated, simple ratios like BUN/K may find renewed utility in predicting complications and personalizing care for patients with acute neurological injuries.

For now, the findings do not change current guidelines for managing subarachnoid hemorrhage or preventing kidney injury. However, they offer a new avenue for risk stratification that warrants further investigation. Clinicians caring for these complex patients may wish to monitor trends in basic metabolic panels not just for isolated abnormalities, but for how they evolve over time.