Neutrophil to Lymphocyte Ratio Predicts Post-Operative Pneumonia in Aneurysmal Subarachnoid Hemorrhage Patients
Aneurysmal subarachnoid hemorrhage (aSAH)—bleeding from a ruptured brain aneurysm into the space around the brain—is a life-threatening condition with high rates of disability and death. Even after successful surgery to repair the aneurysm, patients face serious non-neurological complications, most notably post-operative pneumonia (POP). Up to 37% of aSAH patients develop POP within 30 days of surgery, and it worsens both short- and long-term outcomes. But predicting who will get POP has been challenging—until now.
A 2021 study published in the Chinese Medical Journal suggests a simple blood test could change that. Researchers from Fujian Medical University and affiliated hospitals found that the neutrophil to lymphocyte ratio (NLR)—a measure of inflammation calculated from a routine complete blood count (CBC)—is a strong predictor of POP in aSAH patients.
What Is NLR, and Why Does It Matter?
NLR compares the number of neutrophils (white blood cells that fight infection and inflammation) to lymphocytes (white blood cells that regulate the immune system). A high NLR means more neutrophils and fewer lymphocytes—signs of systemic inflammation or immune dysfunction. For aSAH patients, this imbalance may signal a higher risk of infection, including pneumonia.
The Study: 711 Patients, Clear Results
The team analyzed data from 711 aSAH patients who underwent aneurysm clipping or coiling (surgical repairs) between 2013 and 2018. All patients had a CBC at admission, and researchers tracked who developed POP using modified Centers for Disease Control and Prevention (CDC) criteria (including both confirmed and possible cases).
Key findings:
- 30.4% of patients developed POP (219 out of 711).
- Patients with POP had a significantly higher NLR at admission (14.11 vs. 8.80 in those without POP; P < 0.001).
- Even after adjusting for other risk factors—like age, aSAH severity (measured by the World Federation of Neurosurgical Societies, or WFNS, grade), ventilator use, and treatment type—NLR remained an independent predictor of POP.
- When combined with WFNS grade (a standard measure of aSAH severity), NLR’s predictive power improved—meaning doctors could use both tools to better assess risk.
Who Is at Highest Risk?
The study identified a critical threshold: NLR > 10. Patients with an admission NLR above 10 had a much higher chance of developing POP—regardless of their aSAH severity (WFNS grade). For example:
- Among “good-grade” patients (milder aSAH, WFNS 10 had a 31% lower POP survival rate than those with NLR ≤ 10.
- Among “poor-grade” patients (severe aSAH, WFNS ≥ 3), NLR > 10 doubled the risk of POP.
Even more importantly, NLR was useful even when patients didn’t need long-term mechanical ventilation. For patients on ventilators for more than 24 hours, high NLR still correlated with POP. For those on ventilators for less than 24 hours (or none at all), high NLR was also a red flag.
Why Does NLR Predict POP?
The link between NLR and POP likely stems from immune changes after aSAH. Up to 88% of aSAH patients develop a systemic inflammatory response, driven by the brain’s stress pathways (sympathetic nervous system and hypothalamus-pituitary-adrenal axis). This response increases neutrophils (which fight inflammation but can damage tissues) and decreases lymphocytes (which regulate immunity)—making patients more vulnerable to infections like pneumonia.
NLR also acts as a “marker” for severe aSAH: patients with higher NLR tended to have worse WFNS grades, which are already linked to POP. But unlike WFNS (which requires clinical assessment), NLR is a quick, objective number from a routine blood test—easy for doctors to use in real time.
What Does This Mean for Patients and Doctors?
The biggest takeaway is simplicity: NLR is cheap, widely available, and requires no extra tests. For aSAH patients, an elevated NLR at admission could alert doctors to start preventive measures early—like:
- Pre-operative respiratory training (deep breathing, mouth care).
- Nutritional support to boost immunity.
- Close monitoring for early signs of pneumonia.
For doctors, NLR fills a gap in POP prediction. While WFNS grade helps assess aSAH severity, NLR adds a biological marker that reflects immune status—something clinical grades can’t capture. The study found NLR performed as well as WFNS grade in predicting POP, and combining the two was even better.
Limitations to Consider
Like all research, this study has caveats:
- Single-center data: Results come from one hospital in China, so they may not apply to all populations.
- Possible overestimation of POP: The study included “possible” pneumonia (without clear imaging), which could include non-infectious inflammation. However, main results held when only confirmed cases were analyzed.
- Group vs. individual prognosis: The study looks at trends in groups, not individual patients—more research is needed to refine NLR’s use for personalized care.
The Bottom Line
For aSAH patients, post-operative pneumonia is a common, dangerous complication—but NLR offers a simple way to identify high-risk cases. By checking this ratio at admission, doctors can act early to prevent pneumonia, potentially improving outcomes for one of neurosurgery’s most challenging conditions.
The study was led by Yue Chen, Bao-Qiang Lian, and Lei Peng from the Graduate School of Fujian Medical University and the First Affiliated Hospital of Fujian Medical University, with contributions from the People’s Hospital of China Three Gorges University.
Read the original study:
Chinese Medical Journal (2021). “Neutrophil to lymphocyte ratio is a prognosis factor for post-operative pneumonia in aneurysmal subarachnoid hemorrhage patients.” doi.org/10.1097/CM9.0000000000001304
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