Is it possible for studies comparing Y-set, double-bag, and standard systems of CAPD to be blinded?

Dante Amato and Ramón Paniagua

Unidad de Investigación Médica en Enfermedades Nefrológicas Hospital de Especialidades Centro Médico Nacional Siglo XXI, IMSS, Mexico City, Mexico

Sir,

In their recently published meta-analysis, Daly et al. [1] considered that our work [2] had an acceptable randomisation method, but failed to mask appropriately the study groups. We think that clinical trials comparing peritoneal dialysis devices cannot be performed in a masked way. When Li et al. [3] claimed that their work was done in a single-blind form, we sent a letter [4] commenting that we did not consider it was possible for such a study to be performed in a single-blind form. The compared dialysis devices are so different that we could not imagine a way in which the researchers should manage to keep the patients or the attending physicians unaware of the assigned group. The authors and the editors did not reply, so we assumed that they agreed with our statement.

Let us elaborate a little more on this topic. In single-blind studies the patients are expected to be ignorant of whatever kind of treatment they are receiving, while the attending physician knows it. In double-blind studies both patients and attending physicians are ignorant of the treatment that patients are receiving, or in other words, they do not know to which group the patients were assigned. Patients' blindness allows control of the placebo effect, that is, the effect on the outcomes derived from the patients' beliefs about the assigned treatment effectiveness. Physicians' blindness allows prevention of any changes of treatment (co-interventions) or differences of quality or quantity of delivered care, consciously or sub-consciously derived from the physicians' beliefs about the assigned treatment effectiveness, and also allows the clinicians an unbiased assessment of the studied outcomes. The masking must be of such a kind that resists what a clinician is expected to do, that is, interviewing the patients, performing physical examinations, inspecting the devices used for the treatment, and watching the way in which the patients are using them. When the treatment tested is a pharmacological one, the most popular way of blinding is the use of look-alike placebos. In studies testing devices, ingenuous ways of masking have been reported for gastric freezing [5] or plasmapheresis [6] for instance. In clinical trials, whenever it is possible, a double-blind study should be carried out. However, this is not always possible, sometimes because it is ethically unsustainable to keep the physicians blind for the ongoing treatment, or because the compared treatments are so different that it is not feasible to keep blindness, such as in the present case.

Can Daly et al. explain what kind of masking they were expecting for the studies included in their meta-analysis, and how could it be achieved?

References

  1. Daly CD, Campbell MK, MacLeod AM et al. Do the Y-set and double-bag systems reduce the incidence of CAPD peritonitis? A systematic review of randomized controlled trials. Nephrol Dial Transplant2001; 16: 341–347[Abstract/Free Full Text]
  2. Monteón F, Correa-Rotter R, Paniagua R et al. Prevention of peritonitis with disconnect systems in CAPD: a randomized controlled trial. Kidney Int1998; 54: 2123–2128[ISI][Medline]
  3. Li PKT, Szeto CC, Law MC et al. Comparison of double-bag and Y-set disconnect systems in continuous ambulatory peritoneal dialysis: a randomised prospective multicenter study. Am J Kidney Dis1999; 33: 535–540[ISI][Medline]
  4. Amato D, Paniagua R. Comparison of double-bag and Y-set disconnect systems in CAPD. Am J Kidney Dis1999; 34: 402–403[ISI][Medline]
  5. Zikria BA, De Jesus RS, Cunnick WR, Ferrer JM Jr. Gastric ‘freezing’: A clinical double-blind study. Am J Gastroenterol1967; 47: 208–214[ISI][Medline]
  6. Wallace D, Goldfinger D, Lowe C et al. A double-blind, controlled study of lymphoplasmapheresis versus sham apheresis in rheumatoid arthritis. N Engl J Med1982; 306: 1406–1410[ISI][Medline]

 

Reply

Conal Daly and Marion Campbell

Department of Medicine and Therapeutics, University of Aberdeen, Aberdeen, UK

Sir,

We agree entirely with Amato and Paniagua that masking CAPD delivery-set systems from patients, health care workers and outcome assessors is virtually impossible. Highlighting the absence of masking in the trials included in our paper [1] was certainly not a criticism of these papers. However, we would suggest that even where masking is likely to be virtually impossible, if masking has not been used its absence should, as in their paper [2], be clearly stated. Though masking may be impossible, its absence may still be a cause of bias which should be taken into consideration when either the results of a single trial or heterogeneity between trials included in a meta-analysis are being considered.

References

  1. Daly CD, Campbell MK, MacLeod AM et al. Do the Y-set and double-bag systems reduce the incidence of CAPD peritonitis? Nephrol Dial Transplant2001; 16: 341–347[Abstract/Free Full Text]
  2. Monteon F, Correa-Rotter R, Paniagua R et al. Prevention of peritonitis with disconnect systems in CAPD: a randomised controlled trial. Kidney Int1998; 54: 2123–2128[ISI][Medline]




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