Thermal quantitative sensory testing (QST) is often used in research and clinics to assess the functionality of small nerve fibers. There are two methods commonly used for thermal threshold testing – Limits and Levels. We are often asked which method should be best used. The answer may be: “both!”
But let’s start with the basics.
What is the difference between Limits and Levels?
The Limits method
The Limits test is reaction-time inclusive.
The test starts at a baseline neutral temperature to the skin and either increases (for warm detection and heat pain thresholds) or decreases (for cold detection or cold pain thresholds) at a pre-determined rate.
During the actual temperature change, the patient is required to press the response button as soon as they feel the required sensation.
In this method, the fraction of time it takes for the patient to perceive the sensation, make the decision to press the button, and actually pressing the button is included in the test results. Normative reference data for the Limits test include this very short time lapse. (Read more about using Normative data in our blog)
The Levels method
The Levels test is reaction-time-exclusive.
A stimulus of a certain temperature is presented for a short pre-determined time period, and the patient is then asked whether or not they felt it. The patient can respond by pressing YES or NO on the response unit.
If the stimulus wasn’t felt, the temperature step from baseline for the next stimulus is doubled. If the stimulus was felt, the temperature step for the next stimulus is halved between that of the previous stimulus and baseline. These adjustments are automated by the Medoc Main Station program. In this way, by presenting several stimuli in steps, that close in on each other through smaller and smaller step sizes, the threshold is obtained. This method does not depend on quickness of the patient in pressing the button during the test.
What are the advantages or disadvantages of each method?
The Limits test is a relatively quick test. Usually, several repetitions (between 3-5) are used per thermal modality in order to get an average of the measurements and increase repeatability. Averages and standard deviations for this test are automatically calculated and saved by the Medoc Main Station program.
This specific method is the most often used method for thermal sensation, sometimes called thermal detection thresholds (cold and warm sensation), and thermal pain thresholds (cold pain and heat pain). For the Limits test there is a large body of Normative data for different thermal modalities, age groups, ethnicities and body sites. (van den Bosch, 2017; Yarnitsky, 1994; Rolke, 2006; McKnight, 2010; González‐Duarte, 2016; Blankenburg, 2010).
The method of Levels is usually more time consuming, as a larger number of stimuli are usually needed in order to reach the exact threshold. However, it is not dependent on averaging of multiple tests, rather, it drills down to find the specific first temperature the patient is able to sense. If the patient’s answers are not consistent, the test repeats itself until they are. The Levels method also allows to program a “dummy” stimulus during which the temperature doesn’t change at all, for assessing the consistency of the patient’s responses. Normative data exist for few body sites and population groups (Yarnitsky, 1994; Meier, 2001). The Levels method is especially suitable for patient groups that would have difficulty pressing the response unit in a timely manner, e.g. young children, elderly, persons with mental retardation, or motor control deficits (Defrin, 2004; Meier, 2001).
Most choose the Limits method for pain thresholds. Using Levels for pain thresholds would require to use a long interval between stimuli, in order not to sensitize the patient with supra-threshold stimuli.
Correlation and repeatability between the two methods
There are variable reports on the correlation between the thresholds achieved with the methods of limits and the method of levels. In Complex regional pain syndrome patients, Kemler found little agreement between the two tests at most body sites. (Kemler, 2000)
Sunnergren found lower cold detection thresholds with the method of levels as compared to the method of limits. (Sunnergren, 2010)
The repeatability of these testing methods showed also varying results: Yarnitsky & Sprecher and Bakkers et al. found better repeatability for the method of levels than the method of limits when testing of the feet and hands, Lue et al. found similar values between the two methods on the volar forearm, while Sunnergren found better repeatability for the method of limits when testing intraorally. (Sunnergren, 2010; Bakkers M. F., 2015; Yarnitsky, 1994; Lue, 2017)
Should you choose? Combining Limits and Levels
Some users have chosen to get the best of both works and combine the limits and the levels programs when testing for thresholds. The advantages for using both is that they could use the limits test to know where approximately they should start the levels test, as such, they can shorten the total testing time, while retaining the precision of the reaction-time-independent levels test. (Bakkers M. F., 2015; Reulen, 2003)
Another way to combine the Limits and the Levels methods in your clinical or research work is by using the method of Levels in patients that would have difficulty pressing the response unit in a timely manner, as discussed above, and the method of Limits for the rest of your patient populations.
To read more recommendations on thermal thresholds testing, please refer to the systematic review by Bakkers (Bakkers M. F., 2013).
Meier, P. M. (2001). Quantitative assessment of cutaneous thermal and vibration sensation and thermal pain detection thresholds in healthy children and adolescents. Muscle & Nerve: Official Journal of the American Association of Electrodiagnostic Medicine, 1339-1345.