Quantitative Sensory Testing, or QST, is a group of non-invasive tests that measure how an individual perceives sensations like touch, temperature changes, and pain. The process uses controlled stimuli to determine the precise levels at which a person begins to feel a sensation or when it becomes painful. By turning this subjective patient feedback into measurable data, QST provides a standardized way to assess the function of the somatosensory nervous system, which is responsible for processing these feelings.
The QST Procedure
A QST session involves a series of standardized, non-invasive tests where you report sensory experiences in response to controlled stimuli. A trained examiner will use specialized devices to apply gentle and precise stimuli to specific areas of your skin, often comparing an affected site to a non-affected one. Your active participation is central to gathering accurate data, and you are always in control during the procedure.
For thermal testing, a small, computer-controlled metal plate called a thermode is placed on your skin. This device will gradually get warmer or cooler, and you will be instructed to press a button the moment you first detect the temperature change. The same device can also be used to determine your pain thresholds by continuing to heat or cool the plate until the sensation becomes uncomfortable or painful, at which point you again signal the examiner.
An examiner might use fine, flexible filaments (known as von Frey hairs) to apply a light touch to your skin to determine the slightest touch you can feel. For pressure pain, a device called an algometer is pressed against your skin with slowly increasing force, and you indicate when the pressure becomes painful. Another test assesses your ability to sense vibration by placing a vibrating instrument on your skin, and you report when you feel the sensation and when it stops.
Sensory Pathways Measured by QST
Quantitative Sensory Testing provides detailed information about the function of specific nerve fibers that carry distinct sensory information from the skin to the brain. The tests are designed to assess different types of nerve fibers, which allows clinicians to get a functional picture of these distinct sensory pathways.
Thermal testing specifically evaluates the function of small-diameter nerve fibers. The detection of cold sensations and cold pain are transmitted by thinly myelinated A-delta fibers, while the detection of warmth and heat pain are carried by unmyelinated C-fibers. Because few other objective tests are available for these small fibers, QST is particularly useful for identifying issues related to their function, as abnormalities can indicate nerve damage.
Mechanical testing provides insights into the health of large myelinated nerve fibers. Your ability to detect vibrations, for example, is dependent on large A-beta fibers, which are responsible for transmitting non-painful tactile information. The perception of pinprick sensations involves A-delta fibers, while pressure pain thresholds are used to assess the response of multiple fiber types to strong mechanical force.
Clinical Applications of QST
QST helps diagnose, monitor, and manage conditions affecting the sensory nervous system, particularly chronic pain. The objective data helps characterize a patient’s pain experience and can assist in developing individualized treatment plans.
The application of QST is well-established for neuropathic pain conditions where nerve damage is the source of pain. In diabetic neuropathy, for instance, QST can detect early signs of nerve damage before other symptoms become apparent. It is also used to evaluate conditions like post-herpetic neuralgia (lingering pain after shingles) and chemotherapy-induced peripheral neuropathy. By quantifying the sensory loss or gain, clinicians can better understand the extent of the neuropathy.
QST is also a tool for assessing complex pain syndromes that may not have clear structural causes. For Complex Regional Pain Syndrome (CRPS), the tests can document the sensory abnormalities that characterize the condition. In conditions like fibromyalgia, QST can provide objective evidence of widespread pain sensitivity.
Interpreting QST Results
The data collected during QST is analyzed to create a detailed sensory profile for an individual. This profile is then compared to established normative data from healthy individuals of a similar age and gender. The results are categorized into patterns of sensory “loss,” referred to as negative signs, or sensory “gain,” known as positive signs.
Sensory loss indicates a reduced ability to perceive sensations. This is described with terms like hypoesthesia, which means there is a decreased sensitivity to non-painful stimuli like light touch or vibration. Another negative sign is hypoalgesia, which signifies a reduced sensitivity to a stimulus that would normally be painful. These findings often point toward damage to the nerve fibers responsible for carrying that specific sensory information.
Conversely, sensory gain reflects a state of hypersensitivity. This includes hyperalgesia, which is an exaggerated pain response to a stimulus that is typically only mildly painful. Another form of sensory gain is allodynia, where a person experiences pain from a stimulus that is not normally painful, such as the gentle touch of a cotton swab. These positive signs often indicate that pain-processing pathways in the nervous system have become sensitized.