Department of Clinical Epidemiology & Biostatistics and Centre for Reproductive Medicine, Academic Medical Centre, PO Box 22700, 1105 AZ Amsterdam, The Netherlands
Land et al. (1998) recently reported on the use Chlamydia antibody testing in subfertile patients. The authors state that they compare Chlamydia antibody titres (CAT) and findings at laparoscopy in consecutive patients that attended their fertility clinic, and recommend a cut-off level for CAT of 1:32 or 1:64, depending on the clinical context.
Since in their study the decision for laparospcopy was partially based on the result of the CAT, the series are not as consecutive as the authors want us to believe. The fact that patients with a CAT <8 were much less likely to undergo laparoscopy introduces verification bias, which not only is likely to interfere with the shape of the constructed receiver operating characteristic curves, but also has impact on the interpretation of cut-off levels (Begg and Greenes, 1983).
A second point of concern is that the authors repeatedly dichotomize the CAT in a positive and a negative result, thereby ignoring the option of calculating likelihood ratios for each level of the test. The latter is considered to be one of the major advantages of the use of such test parameters (Sackett et al., 1991). We recalculated likelihood ratios for several levels of the CAT from the data provided by Land et al. (1998) using the most severe definition of tubal disease, i.e. extensive periadnexal adhesions and/or occlusion of both tubes (Table I
).
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Notes
1 To whom correspondence should be addressed
References
Begg, C.B. and Greenes, R.A. (1983) Assessment of diagnostic tests when disease verification is subject to selection bias. Biometrics, 39, 207215.[ISI][Medline]
Land, J.A., Evers, J.L.H. and Goossens, V.J. (1998) How to use Chlamydia antibody testing in subfertility patients. Hum. Reprod., 13, 10941098.[Abstract]
Mol, B.W.J., Swart, P., Bossuyt, P.M.M., Van der Veen, F. (1997) Is hysterosalpingography an important tool in predicting fertility outcome? Fertil. Steril., 67, 663669.[ISI][Medline]
Sackett, D.L., Haynes, R.B., Guyatt, G.H. and Tugwell, P. (1991) Clinical Epidemiology: A Basic Science for Clinical Medicine. Little Brown and Company, Boston, Toronto, London, UK.
Department of Obstetrics and Gynaecology, Academisch Ziekenhuis Maastricht
Department of Medical Microbiology, Academisch Ziekenhuis Maastricht PO Box 5800, 6202 AZ Maastricht, The Netherlands
Dear Sir,
We note the work of Mol et al. (Mol and Bossuyt, 1995; Mol et al., 1996a
,b
,c
; 1997a
,b
; Mol and Van der Veen, 1997
) and read with interest their letter to the editor regarding our recent Chlamydia manuscript (Land et al., 1998
). We appreciate their comments, and agree that verification and selection bias are hard to prevent in clinical studies, unless one is prepared to perform the complete fertility investigation of a patient on a single day. Another example for this selection bias can be found in the second part of their present letter (Table II), where the authors refer to their own previous study (Mol et al., 1997c
) in which they failed to do an hysterosalpingography (HSG) in all patients at the very moment they entered their clinics but still made inferences regarding the predictive value of HSG.
Regarding their second concern: research is data reduction, and we therefore would like to leave it up to the reader whether he prefers our dichotomized single cut-off levels or the individual likelihood ratios per test result, as proposed in their present letter to the editor.
1 To whom correspondence should be addressed
References
Land, J.A., Evers, J.L.H. and Goossens, V.J. (1998) How to use Chlamydia antibody testing in subfertility patients. Hum. Reprod., 13, 10941098.[Abstract]
Mol, B.W.J. and Bossuyt, P.M.M. (1995) Surgical treatment of ectopic pregnancy. [Letter to the editor.] Lancet, 346, 638639.[ISI][Medline]
Mol, B.W.J. and Van der Veen, F. (1997) A study of ruptured tubal ectopic pregnancy. [Letter to the editor.] Obstet. Gynecol., 90, 866867.
Mol, B.W.J., Pajkrt, E., Van Lith, J.J. and Bilardo, C.M. (1996a) Screening for fetal trisomies by maternal age and fetal nuchal translucency thickness at 10 to 14 weeks of gestation. [Letter to the editor.] Br. J. Obstet. Gynaecol., 103, 10511052.[ISI][Medline]
Mol, B.W.J., Van der Veen, F., Redekop, K. and Bossuyt, P.M.M. (1996b) The value of diagnostic tests in infertility. [Letter to the editor.] Ned. Tijdschr. Geneeskd., 140, 279280.
Mol, B.W.J., Van der Veen, F. and Bossuyt, P.M.M. (1996c) Time to pregnancy after ectopic. [Letter to the editor.] Fertil. Steril., 66, 172173.[Medline]
Mol, B.W.J., Hajenius, P.J., Ankum, W.M. et al. (1997a) Comparative costs of methotrexate and laparoscopic surgery. [Letter to the editor.] Hum. Reprod., 12, 16031604.[ISI][Medline]
Mol, B.W.J., Van der Veen, F., Lijmer, J. and Bossuyt, P.M.M. (1997b) Observer variation and clinical decision making. [Letter to the editor.] Fertil. Steril., 68, 381384.[ISI][Medline]
Mol, B.W.J., Swart, P., Bossuyt, P.M.M. and Van der Veen, F. (1997c) Is hysterosalpingography an important tool in predicting fertility outcome? Fertil. Steril., 67, 663669.[ISI][Medline]