Structural validity of the Boston Carpal Tunnel Questionnaire and its short version, the 6-Item CTS symptoms scale: a Rasch analysis one year after surgery

Abstract

Background: The Boston Carpal Tunnel Questionnaire (BCTQ) and its shorter version, the Six-Item Carpal Tunnel Symptoms Scale (CTS-6), are widely used for assessing function and/or symptoms in patients with carpal tunnel syndrome. This study examined the structural validity of the BCTQ and CTS-6 among patients who had undergone surgery for treatment of carpal tunnel syndrome.

Methods: The data for this cross-sectional analysis were obtained from 217 adult patients who had undergone carpal tunnel release surgery 1 year earlier. All patients completed the CTS-6, Symptom Severity Scale (SSS) and Functional Status Scale (FSS) of the BCTQ at 12 months after surgery. The Rasch Measurement Theory (RMT) was applied to investigate the unidimensionality, residual correlation, differential item functioning, scale coverage/targeting, and person separation of the CTS-6, SSS and FSS of the BCTQ.

Results: The FSS showed unidimensionality and good scale and item fit. All items showed ordered response category thresholds. Eight of the FSS items displayed differential item functioning favoring age or gender. The multidimensional structure of the CTS-6 was absorbed by creating a testlet for frequency of symptoms or testlets for pain and numbness. The testlets supported unidimensionality in the BCTQ SSS. One item in the CTS-6 and two items in the BCTQ SSS showed differential item functioning favoring age or gender. Four items in the BCTQ SSS and two items in the CTS-6 exhibited disordered response category thresholds. Merging of the relevant response categories led to ordered response category thresholds. The person separation indices were 0.73, 0.86 and 0.77 for the CTS-6, BCTQ SSS and FSS, respectively.

Conclusions: Based on the RMT analysis, the CTS-6 has superior psychometric properties compared to the BCTQ SSS in surgically treated patients. The CTS-6 might be more accurate when separated into item sets measuring pain or numbness. The FSS of the BCTQ has acceptable construct validity, although gender differences at some ages were observed in responses.

Keywords: Boston Carpal Tunnel Questionnaire; Carpal Tunnel Release; Psychometrics; Rasch analysis; Six-Item Carpal Tunnel Symptoms Scale.

Fig. 1

Differential item functioning by gender for item 6 (“How often did the following symptoms in your hand wake you up at night? - numbness or tingling”) of the CTS-6. Note in figure: ‘Some person location values … decrease minimum location’ and ‘Person location’

Fig. 2

Person-item threshold distribution map for the CTS-6. Note in figure: Grouping set to interval length of 0,20, making 70 groups ‘Under 60yrs’ ‘At least 60yrs’

Fig. 3

Response category threshold curve showing disordered thresholds for item 3 (“How severe are the following symptoms in your hand? –numbness or tingling at night”) in the CTS-6. The response options were 0= “None”, 1= “Mild”, 2= “Moderate”, 3= “Severe”, 4= “Very severe”. Note in figure ‘Person location’

Fig. 4

Response category threshold curve showing disordered thresholds for item 6 (“How often did the following symptoms in your hand wake you up at night? –numbness or tingling”) in the CTS-6. The response options were 0= “Never”, 1= “Once”, 2= “2 or 3 times”, 3= “4 or 5 times”, 4=“More than 5 times”. Note in figure ‘Person location’

Fig. 5

Response category threshold map showing ordered thresholds after merging the response categories of items 3 and 6 in the CTS-6

Fig. 6

Differential item functioning favoring age in item 5 (“How long on average does an episode of pain last during the daytime?”) of BCTQ Symptom Severity Scale. Note in figure: ‘Some person location values … decrease minimum location’ and ‘Person location’

Fig. 7

Differential item functioning by gender in item 7 (“Do you have weakness in your hand or wrist?”) of the BCTQ Symptom Severity Scale. Note in figure: ‘Some person location values … decrease minimum location’ and ‘Person location’

Fig. 8

Person-item threshold distribution map for the BCTQ Symptom Severity Scale. Note in figure: Grouping set to interval length of 0,20, making groups’ ‘Under 60yrs’ ‘At least 60yrs’

Fig. 9

Response category threshold curve showing disordered thresholds for item 4 (“How often do you have hand or wrist pain during daytime?”) in the BCTQ Symptom Severity Scale. The response options were 0= “Normal”, 1= “1-2 times / day”, 2= “3-5 times / day”, 3= “More than 5 times”, 4= “Continuous”. Note in figure: ‘Person location’

Fig. 10

Response category threshold map showing ordered thresholds after merging the response categories in items 4 (“How often do you have hand or wrist pain during daytime?”), 5 (“How long on average does an episode of pain last during the daytime?”), 7 (“Do you have weakness in your hand or wrist?”) and 11 (“Do you have difficulty with the grasping and use of small objects such as keys or pens?”) of the BCTQ Symptom Severity Scale

Fig. 11

Person-item threshold distribution map for the BCTQ Functional Status Scale. Note in figure: Grouping set to interval length of 0,20, making groups’ ‘Under 60yrs’ ‘At least 60yrs’

Fig. 12

Response category threshold map showing ordered thresholds in the BCTQ Functional Status Scale