Discussion
In Turkey, standardized QoL tool for the assessment of disease burden in
CVID patients is lacking. In the current study which had high response
rate and positive response from CVID patients we validated the Turkish
version of CVID QoL questionnaire and its psychometric properties. It
showed excellent reliability, good content validity and reproducibility.
Concerning reliability, our results revealed that all items had
excellent internal consistency (>0.9) and 2 subscales, EF
and RF exhibited good internal consistency as well (>0.7).
These findings are in agreement with the results (0.82, 0.84) of the
original Italian version 10 and similar ( 0.91, 0.77)
to the Norwegian cultural adaptation study 19. The GSS
subscale did not achieve the acceptable internal consistency. It
consists of only 4 items, 2 of them are related to the diarrhea, 1 skin
diseases and 1 dietary change and these 4 elements were not very related
to each other. This may be one of the reasons for the low internal
consistency. When we consider only two items (4 and 14) which directly
deal with bowel symptoms, it exhibited good internal consistency similar
to the findings of the Norwegian study 19. Another
possible reason we considered was that our sample group was small to
establish construct validity 25. In addition,
cutaneous problems are not seen as often as gastrointestinal
manifestations. Generally autoimmune skin problems and case based
cutaneous diseases are seen 26-28. On the other hand,
Ballow et al. developed and published a new disease specific tool for
primary antibody deficiencies. It did not include any question about
skin problems 29. Therefore, we may consider that
dermatologic features do not have an important impact on QoL of CVID
patients, but more comprehensive studies are necessary to indicate this.
Additionally, similar to the findings from the index study and cultural
adaptation 10, 19 test–retest reliability results
indicated that the Turkish CVID QoL scale (CVID QoL TR) also exhibits
excellent short-term stability. This indicated that outcomes from the
CVID QoL TR were reproducible, supporting its potential use as a
patient-reported outcome tool.
Content validity is the ability of a tool to determine the area of
interest and the conceptual definition of a structure25. During the determination of content validity, we
found our CVI was acceptable. But the content validity ratio could not
reach the value of 0.7 in 6 items for at the first stage. Minor changes
were then made in the 6 items and the main structure was maintained. The
last version of tool was approved. We considered that these findings
contributed to the content validity.
Convergent validity assesses the extent to which a questionnaire/ tool
measures what it is designed to measure 30. It is
estimated by correlating its items with other validated questionnaires
measuring the same or similar constructs. To examine the convergent
validity of the CVID-QoL-TR, we used SF-36 as a comparative tool. SF-36
is a well-known general QoL scale, translated and validated in Turkish
language and used in various diseases 31, 32. Good
correlations were found between QoL global, EF and RF subscales of the
CVID-QoL-TR with certain items of the SF-36. CVID QoL scores correlated
strongly with both SF-36’s physical and mental health domains. Quinti et
al. showed good convergent validity for the EF and RF subscales
correlating with conceptually similar dimensions of SF-3610. Andersen et al. reported the similar findings
with/to WHQOOL BREF19. Discriminant validity is a
statistical concept assessing the ability of a tool/questionnaire to
detect true differences and discriminate between the other tools or
changes. It indicates that the two things/measure that should not be
related are actually irrelevant 25. Our results showed
that the QoL, EF, RF scores were higher in the patients complaining of
more than one infection within 3 months before the study. Quinti et al
reported that the frequency of infections both within 3 months and 12
months before the study had an impact on the quality of life. We did not
observe this association within 12 months before the study. We can
speculate that this might be related to the questionnaire seeking
answers to questions about the last 3 months and 12 months is a longer
duration to recall.
Factor analysis is a multivariate statistics that obtains to find a
small number of conceptually significant new variables (factors,
dimensions) by combining a large number of related variables intended to
measure the same structure or a particular property33,
34 More accurate factor analyses stated that sample size should have at
least 3-5 times more number of items 35. In the
current study, we could not verify the factor analysis since our sample
size did not have large number of participants. However, we could
perform factor analysis for GSS and RF subscale because they have 4 and
9 items respectively. We observed GSS subscale item 4 and14 related with the bowel symptoms were distinguished from theitem 2 ‘dietary changes’ and 26 ‘skin symptoms. In RF
subscale, item 11 ‘run out of medications’ and item 16 ‘as
contagious’, item 6 ‘cough’ and item 25 ‘limitation of
leisure activity’ were distinguished from the other items in the RF
dimension. But EF and QoL had more items than 50-participant-group could
verify these factors. Although factor analysis does not confirm 3
factors. Good correlation with SF 36, reliability, reproducibility and
high response rate showed us that CVID QOL TR is a useful scale. We can
believe that factor analysis can be re-evaluated as the instrument will
be used in the future.
We observed that being female were negatively associated with QoL. This
finding was similar to the information of other CVID QoL
studies10, 16, 18. Receiving IVIG treatment was the
second factor associated with poor QoL, it was also consistent with the
previous studies10, 16. We observed better quality of
life in the patient with more than 13 years of education similar to the
findings of the Italian and Norwegian group. We did not observe any
association between BMI, age and QoL in our study. It might be related
to the ethnic differences. Generally, we could not compare directly the
findings of our study between the Italian and Norwegian study groups.
Our QoL scores did not normally distributed, but the Italian and
Norwegian groups showed that their findings were normally distributed
(Table 3.) Our study group achieved similar mean CVID QoL scores with
Norwegian group while higher than Italian group. 43.7 % of all replies
were 0 and our floor and ceiling effects showed better QoL. Since these
effects were not evaluated in the index study so we could not compare
the findings totally. Differences in the results of our study group
between the other groups may be explained with the variation in the
demographic features of the study groups. Our study group had higher
proportion of male and younger patients as well as the education levels
of our patients were lower than the participants of other study groups.
Furthermore, it might be associated with low socioeconomic status or
other cultural differences that could not be differentiated in the
disease specific tools. CVID QoL instrument is a disease specific
questionnaire. It could not measure the impact the social requirements
or economic or psychologic situations. Finally, we believe it is not
suitable for comparison.
Our study had some limitations. One of them was the low number of adult
CVID patients included in the study though we are one of the largest
centres in Turkey. Therefore, the analysis could not be properly done to
verify the factor analyses and structural validity. Second, there is
also the possibility of recall bias, since patients were asked to report
on their health in the past 3 months.
In conclusion, CVID disease specific questionnaire is necessary to
better evaluate the disease burden on the patients Our study indicated
that Turkish version of CVID QoL questionnaire was a reliable, useful
and valid instrument for the measuring of quality of life in CVID
patients.
It is recommended to investigate its stability by applying it to larger
patient groups and further consideration on factor analysis. In addition
to that, future evaluation of QoL in CVID either with this CVID QoL TR
in other Turkish patients or also other translations to other languages
can facilitate the improve the knowledge about CVID disease burden on
individually.