The ‘sniff test’: how a sense of smell could help predict recovery after severe brain injury
For families supporting a loved one with a severe brain injury, one of the most difficult challenges is coping with uncertainty. Understanding whether someone is aware of their surroundings, and what their chances of recovery may be, is crucial for making treatment decisions and planning for the future.
In recent years, researchers have made significant progress in identifying new ways to assess consciousness in patients with severe brain injuries. One of the most promising developments is a simple test based on a patient’s sense of smell, often referred to as the ‘sniff test’, or olfactory testing in clinical settings. Studies suggest that a patient’s response to different odours may help clinicians determine whether the patient is in a minimally conscious state, predict their likelihood of regaining consciousness, and even provide insight into their long-term survival prospects.
Why an accurate diagnosis matters
Following a severe brain injury, it can be extremely difficult for medical professionals to determine whether a patient is unresponsive or minimally conscious. Research suggests that misdiagnosis rates can be as high as 40%, highlighting the challenges involved in assessing consciousness.
Accurate diagnosis is important because these two conditions can have very different outcomes.
An unresponsive patient shows no awareness of themselves or their environment. By contrast, a person in a minimally conscious state demonstrates some level of awareness, either of themselves, their surroundings, or both, even though that awareness may be significantly impaired.
A minimally conscious state often follows a coma or vegetative state. For some people it is a temporary stage on the path to recovery from brain injury, while for others it may become a long-term condition. Identifying the difference as early and accurately as possible helps doctors develop appropriate treatment and pain management plans, while also providing families with a more realistic understanding of their loved one’s prognosis.
The original ‘sniff test’ study with striking results
In 2020, researchers from the University of Cambridge’s Department of Psychology and the Weizmann Institute of Science in Israel explored whether a patient’s sense of smell could reveal important information about their level of consciousness and potential for recovery.
The study involved 43 patients with severe brain injuries who showed either very limited signs of awareness or no apparent awareness of the external world. Researchers exposed patients to different odours, including shampoo, rotten fish and a neutral control with no smell, while monitoring their breathing through a nasal cannula.
The theory was simple: healthy individuals naturally change the way they breathe when they encounter different smells, often taking shorter or shallower sniffs in response to unpleasant odours. Researchers wanted to know whether patients with severe disorders of consciousness would display similar unconscious responses.
The results were striking.
Every patient who demonstrated a measurable sniff response later regained consciousness. Furthermore, 91% of those patients were still alive three years after their injury. In contrast, among patients who showed no response to the odours, 63% had died within three and a half years.
Anat Arzi, who led the research, explained that some patients who appeared to be in a vegetative state nevertheless demonstrated a sniff response and later progressed to at least a minimally conscious state. In several cases, this response was the earliest indication that recovery was possible, appearing days, weeks or even months before any other signs became visible.
The findings suggested that measuring a patient’s response to smells may offer a simple way of assessing brain function in areas associated with consciousness and arousal. Importantly, the sniff test could provide valuable information even when more traditional assessments fail to detect signs of awareness.
Building on the research
Since the publication of the original study, further research has explored the potential role of smell-based testing in assessing consciousness.
The 2022 clinical trial – familiar smells help distinguish levels of consciousness
A double-blind, randomised clinical trial published in 2022 examined 23 patients with disorders of consciousness. Researchers assessed behavioural responses to familiar, neutral, unpleasant and odourless stimuli.
The study found a significant difference between patients in a minimally conscious state and those in an unresponsive wakefulness syndrome (formerly known as a vegetative state) when familiar smells were presented. This supported the idea that olfactory testing could help clinicians distinguish between different levels of consciousness.
However, unlike the original sniff test study, researchers did not find the same strong link between smell responses and long-term recovery. The authors noted that factors such as the types of odours used, testing methods and patient characteristics may influence results and require further investigation.
The 2023 EEG study – brain activity patterns detected
A further study published in Frontiers in Neuroscience in 2023 combined smell testing with electroencephalogram (EEG) monitoring to examine brain activity in 28 patients with disorders of consciousness.
Researchers presented pleasant, unpleasant and neutral odours while simultaneously recording brainwave activity. The results showed that patients who responded to smells generally displayed higher levels of consciousness.
The study also identified distinct patterns of brain activity in patients who did not respond to odours, providing researchers with additional insight into the neurological processes associated with unconscious states.
Perhaps most importantly for families, the study found that 62.5% of patients who responded to odours showed signs of recovering consciousness within three months, compared with just 16.7% of patients who did not respond.
Together, these studies suggest that olfactory testing may offer valuable information when used alongside other established methods of assessing consciousness.
Looking ahead
Interest in smell-based assessments continues to grow within the medical community. Researchers increasingly view olfactory testing as a useful addition to existing diagnostic tools, rather than a replacement for them.
Clinical guidelines and neurological reviews have begun to recommend incorporating smell assessments into broader, multidisciplinary evaluations of patients with severe brain injuries. Meanwhile, ongoing international research is working to standardise testing methods and confirm results across larger patient groups.
Currently, an active clinical trial in France is investigating how olfactory stimulation can be used to predict recovery in comatose patients. Researchers hope this work will help establish consistent protocols that can be adopted more widely in hospitals around the world.
While further validation is still required, the growing body of evidence suggests that something as simple as a patient’s response to smell may provide important clues about consciousness, recovery and survival.
For families living with the devastating effects of a severe brain injury, uncertainty can be one of the most difficult aspects of the journey. Advances in research, such as the sniff test, offer encouraging possibilities for improving how doctors assess consciousness and predict recovery.
Although no single test can provide all the answers, olfactory assessments may become an important part of a wider diagnostic approach, helping clinicians identify signs of awareness that might otherwise be missed. As research continues, these simple, non-invasive tests could improve diagnostic accuracy, support treatment decisions and provide families with clearer information during an incredibly challenging time.
Most importantly, studies in this area demonstrate that hidden signs of awareness can sometimes exist even when they are not immediately visible.
