Affiliations: University of Tsukuba, Tsukuba-City, Japan ,
Veterans General Hospital-Taipei, Taipei, Taiwan
Correspondence to: Hiroaki Satoh, MD, Division of Respiratory Medicine, Institute of Clinical Medicine, University of Tsukuba, Tsukuba-City, Ibaraki, 305-8575, Japan; e-mail: firstname.lastname@example.org
To the Editor:
We read with interest the report by Lee and colleagues (December
1998)1on the usefulness of sialyl stage-specific
embryonic antigen-1 (sialyl Lewis X-i antigen). We also have studied
and published articles on the clinical utility of the
antigen.2–5 The determination of the cutoff value for
their study according to the mean ± SD seems inappropriate because
confirmation of the “normal distribution” is not warranted. We
believe that receiver operating characteristic (ROC) curve analysis is
one of the suitable methods for determination of the cutoff value.
We can share the authors’ observation that “pleural sialyl Lewis X-i
antigen levels > 265 U/mL were considered to indicate a diagnosis of
adenocarcinoma of the lung.” Our results showed, however, that
levels ≥ 214 U/mL were found only in patients with
adenocarcinomas that had been proven by cytologic examinations of
pleural effusions. This level accounted for 95%
specificity. Our study included data from malignant pleural effusions
resulting from metastatic adenocarcinomas of organs other than the
We would like to reply to the comments on our article (December
1998)1 by Satoh and colleagues. They suggest that the
receiver operating characteristics (ROC) curve analysis appears to be
one of the suitable methods to be used for the determination of a
cutoff value, unless normal distribution of the data is confirmed.
To the best of our knowledge, there is no standard method for the
determination of a cutoff value for a tumor marker. The selection of a
cutoff point is dependent mainly on clinical significance. To avoid
false-positive results, the highest value obtained from benign cases
may be chosen as a cutoff point. In this setting, the specificity
appears to be 100%, but at the expense of sensitivity. To increase the
sensitivity, a lower value may be used as the cutoff value, but at the
expense of specificity.
In our study, the value given by the mean + 2SD of pleural fluid
sialyl stage-specific embryonic antigen (SSEA)-1 was selected as a
cutoff point because the data were found to be normally distributed in
each subgroup (groups 3 to 6) of our benign cases.1 It is
plausible that the data are normally distributed when these cases
(groups 3 to 6) are pooled as one group. Unfortunately, the data are
almost but not quite normally distributed. The ROC curve
analysis is used as suggested to determine the cutoff value: 295 U/mL
instead of 265 U/mL. When 295 U/mL is used as the cutoff point, the
sensitivity and specificity are 64% and 96%, respectively. The
sensitivity and specificity are 64% and 95%, respectively, when 265
U/mL is used as the cutoff point. In terms of sensitivity and
specificity, the ROC curve analysis adds little value. This supports
the notion that the method used in our study for the determination of
the cutoff value is adequate.
The clinical significance of the cutoff values selected by either
method, mean + 2SD or ROC curve analysis, is comparable in our study.
However, we agree that the ROC curve analysis is one of the suitable
methods to be used to determine the cutoff point of pleural fluid
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