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Thermal testing: what is the norm? A guide to using normative data

This blog explores:

table of thermal sensory detection thresholds
Normative data example

What is normative data?

Normative data is a large amount of values gathered from people who, in this context, are known to be healthy. This data is gathered using standardized methodology and equipment. In gathering quantitative sensory test results (QST) from healthy subjects in defined age and sex groups, a good comparison to a “healthy standard” can be made. Normative data may differ for specific ethnic backgrounds, therefore, several labs around the world have endeavored to gather data from the local population in their country.[1,2]

How is normative data used?

Normative data is used in order to compare the results of a specific test from a subject of interest to age and gender matched healthy controls. If a person’s average sensory thresholds fall within the 95% confidence interval of the normative data gathered for their peers, that person’s thresholds would be considered within normal range. However, if a person’s thresholds fall outside of these values, this may possible point to a sensory deficit. This, however, should be considered together with that person’s anamnesis and clinical/laboratory examination.

Does Medoc’s software include normative data?

Yes, Medoc Main Station (MMS). Medoc’s software includes normative data from three different sources, each with its own unique qualities:

1. Blankenburg, et al. (2010). Reference values for quantitative sensory testing in children and adolescents: developmental and gender differences of somatosensory perception.[3]

Blankenburg et al., collected normative data from children and adolescents in the ages of 6 to 16. They collected data in Germany according to the protocol of the German research network on neuropathic pain (DFNS), from 176 healthy subjects in total. They tested on the following sites: mandible, medial forearm, dorsomedial foot.

2. Yarnitsky & Sprecher (1994). Thermal testing: normative data and repeatability for various test algorithms.[4]

Yanitsky & Sprecher collected normative data in Israel from 106 adults, males and females between the ages of 20-79. The normative data was collected using both the reaction-time inclusive Limits method, and the reaction-time exclusive Levels method. There are normative data available for the hand and the foot.

3. Hafner, et al. (2015). Thermal quantitative sensory testing: a study of 101 control subjects.[5]

Hafner et al. collected normative data from 101 healthy men and women in the ages between 21-70, in Australia. They used the Limits method for cold detection and warm detection, and tested the thenar eminence and the dorsolateral foot. Notably, two TSA-II devices were used, but they found no difference in testing using one TSA-II vs. another TSA-II device.

In the MMS software it is possible to easily switch between the sources, via Normative Data utility, and use the normative values most suitable to the tested subject.

How can I make sure that I can compare my data to normative data?

In order to compare your data to normative data it is imperative to make sure that:

1. You use the same technology

The technology may affect the curve with which the temperature changes, the rate with which the temperature may change, the reliability and repeatability of each test, and the calibration of the temperature exerted by the probe (thermode).

Medoc’s devices are calibrated prior to leaving the plant, and customers can check calibration, and if necessary re-calibrate the thermode using an external calibration kit.

Finally, all of Medoc’s thermal devices use the same core technology and can therefore be compared. A scientific comparison has been made between Medoc’s Q-Sense device and Medoc’s TSA-II device, and they were found to be comparable.[6]

2. You use the same thermode size

Differences in size of the stimulus area (thermal contact plate) will affect sensation due to differences in spatial summation. It is therefore important that the same thermode size will be used.

3. You use the same program parameters

Program parameters may be: the baseline temperature the stimulation starts from, the rate of temperature change, the number of stimuli, and the time between stimuli (Inter-Stimulus Interval, ISI).

The baseline temperature is the temperature each stimulus starts from. The baseline temperature should be felt as neutral, i.e. not cold nor warm. The standard protocol baseline temperature is 32°C, as it reflects the temperature of the skin.

The rate of temperature change is significant specifically in reaction-time inclusive tests like the “Limits” test. The reaction time from sensing the temperature change until pressing the button to stop the stimulus is included in the results. Therefore, performing the test at a faster or slower pace would significantly affect the results. The rate you need to use in Limits test to be able to rely on existing normative data is 1°C/sec, the rate with which the normative data was collected.

The Inter-Stimulus Interval (ISI) allows the skin to regain back its baseline temperature, and the sensory nervous system to return to a steady-state function. There is importance in standardizing the ISI, since one would like to start every test from a level approximating steady state, but with retaining the test subject’s concentration for the test.

In terms of the number of stimuli, using several stimuli allows to increase repeatability of the test results by taking an average of these stimuli. Usually, at least 3 stimuli per testing modality are used, but some users choose more. This is often dependent of considerations of time expenditure, and cognitive load for the test subject. In the MMS software this parameter does not affect the use of normative data.

4. You test on the same body site for which normative data exist

Different body sites have different innervation density and composition, for instance, hairy and glabrous skin have different innervation composition in terms of A-delta and C-fibers. Additionally, the distance of the body site to the brain may significantly affect the results of the thermal test due to the longer way the signal has to travel.

Do you have normative data for pain thresholds?

Currently no pain thresholds are implemented. However, there are normative data available through published works which can be imported into the MMS.

Can I use my own normative data?

This is definitely possible. Normative data can be uploaded into MMS and imported into the program, so you can implement your own normative data in the software.

Is using normative data the only way to get meaningful information out of QST about a patient/subject?

Not necessarily. In using QST, for instance in the case there are unilateral complaints, one could compare one site to its contralateral hemi-side.[7,8] QST could also be performed before and after an intervention. Some researchers choose to collect data from controls of the same racial, ethnic, or gender background with their own specific protocol.[1,2,9,10]



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