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BMJ小小統計問題(22)

前言：

新主題，” Randomised controlled trials (RCT)”專論，相信RCT很多人一定不陌生，是臨床研究最基本的設計方法。最根本，才是最重要。不少朗朗上口的概念在進一步探究下，可能發現目前所知，原來似是而非的。

題目：

研究人員評估持續呼吸道正壓對中重度阻塞性睡眠呼吸暫停病人血壓的影響，這些病人最近被診斷為系統性高血壓，但未得到治療。採用24小時動態監測測量血壓。採用雙盲隨機對照試驗設計。對照治療是假性持續呼吸道正壓通氣，在非常低的壓力下進行。共有169名病人被分配到持續呼吸道正壓，171名病人被分配到對照組治療[1]。

主要結果是平均24小時動態血壓監測值在3個月後的變化。分析是根據治療意向(intention to treat, ITT)原則進行的。持續呼吸道正壓與系統性高血壓和阻塞性睡眠呼吸暫停病人的血壓顯著降低。

下列何者最能描述ITT分析原則？

a)僅當病人打算在隨機化後接受治療時才納入分析

b)根據兩組病人最終接受的治療進行比較

c)兩組病人在最初分配治療的基礎上進行比較

d)參與者僅在完成最初分配的治療後才被納入分析

e)病人只有在開始隨機分配的治療後才被納入分析

答案為(C)。

詳細說明：

意向治療分析包括最初使用隨機分配的治療組中的所有試驗參與者。這與病人是否開始接受分配的治療、是否隨後退出試驗、是否不遵守分配的治療方案或是否接受了與最初分配的治療不同的治療無關。意向治療分析是分析臨床試驗的標準方法。

意向治療分析保持了試驗參與者隨機分組後，治療組別在潛在干擾因素方面最初的可比較性。

干擾因素是指可能同時影響治療組別(X)和結果(Y)量測的因素，如人口統計學、預後因素和其他可能影響個人參與或退出試驗的特質。若樣本數足夠大，在研究結束時，治療組別間的任何差異都將是所接受治療差異的結果，而非治療組之間潛在的干擾所造成。治療意向分析的目的主要是一種實用主義，因為它反映了臨床實踐中會發生的事情。並非所有的治療都是可接受的或耐受性良好的，病人經常不能開始、完成或繼續他們的治療方案。因此，意向治療分析提供了對治療實際效果的評估。

試驗中相當多的病人未能完成分配給他們的治療。在169名接受持續呼吸道正壓治療的患者中，137名（81.1%）完成了試驗，而171名接受假性治療的病人中有135名（78.9%）完成了試驗。未完成試驗的原因包括拒絕一開始分配的治療、對治療缺乏耐受性、違反方案和技術問題。

原文：

Researchers assessed the effects of continuous positive airway pressure on blood pressure in patients with moderate to severe obstructive sleep apnoea and recently diagnosed but untreated systemic hypertension. Blood pressure was measured using 24 hour ambulatory monitoring. A double blind randomized controlled trial study design was used. Control treatment was sham continuous positive airway pressure delivered at a very low pressure. A total of 169 patients were assigned to continuous positive airway pressure and 171 to sham treatment.[1]

The primary outcome was the change at three months from baseline in mean 24 hour ambulatory blood pressure monitoring values. The analysis was undertaken by the principle of intention to treat. Continuous positive airway pressure was associated with a statistically significant reduction in blood pressure in patients with systemic hypertension and obstructive sleep apnoea.

Which one of the following statements best describes the principle of intention to treat analysis?

a) Patients are included in the analysis only if they intend to be treated after randomisation

b) Two groups of patients are compared on the basis of the treatment they eventually receive

c) Two groups of patients are compared on the basis of the treatment they were originally allocated

d) Participants are included in the analysis only if they complete the treatment originally allocated

e) Patients are included in the analysis only if they start the treatment allocated after randomisation

Answer

Answer c is the best description.

Reference:

[1] Duran-Cantolla J, Aizpuru F, Montserrat JM, Ballester E, Teran-Santos J, Aguirregomoscorta JI, et al; on behalf of the Spanish Sleep and Breathing Group. Continuous positive airway pressure as treatment for systemic hypertension in people with obstructive sleep apnoea: randomised controlled trial. BMJ 2010;341:c5991.