抉擇醫療處置

抉擇醫療處置

• 定性評估

1. 病情嚴重度
2. 病情可治療的程度
3. 診斷工具的風險
4. 治療風險

• 定量評估：檢查前可能性（依文獻）、可能比率（依文獻）、檢查後可能性。

1. 檢查前機率（Pretest Probability）：檢查前預測疾病之可能性。→檢查前勝算（Pretest Odds）= (Pretest Probability / (1 - Pretest Probability)
2. 可能性比率（Likelihood Ratio）：診斷工具的強度（由敏感度、特異度算來），數理上通常以「陽性且有病 ÷ 陽性但沒病」代表「陽性診斷工具的強度」，如：陽性診斷工具強度3.5，代表9人中工具誤判2人→陽性有病7人，陽性沒病2人。
3. 檢查後勝算（Posttest Odds） = Pretest Odds  × Likelihood Ratio
4. 檢查後機率（Post-test Probability） = Posttest Odds / (Posttest Odds + 1)

【例子】
　　A. 檢查前可能性＝25%，可能性比率＝6，則檢查後可能比率＝？
　　　檢查前勝算：25/(100-25) = 1/3
　　　檢查後勝算：1/3 × 6 = 2
　　　檢查後機率：2/(1+2) = 2/3 → 67%

　　B. An individual was screened with the test of fecal occult blood (FOB) to estimate the
　　　probability for that person having the target condition of bowel cancer, and it fell out positive
　　　(blood were detected in stool). Before the test, that individual had a pre-test probability of 
　　　having bowel cancer of, for example, 3% (0.03), as could have been estimated by
　　　evaluation of, for example, the medical history, examination and previous tests of that
　　　individual.
　　　
　　　The sensitivity, specificity etc. of the FOB test were established with a population sample of
　　　203 people (without such heredity), and fell out as follows:

　　　From this, the likelihood ratios of the test can be established:
　　　Likelihood ratio positive = sensitivity / (1 − specificity) = 66.67% / (1 − 91%) = 7.4
　　　Likelihood ratio negative = (1 − sensitivity) / specificity = (1 − 66.67%) / 91% = 0.37
　　　Pretest probability (in this example) = 0.03
　　　Pretest odds = 0.03 / (1 - 0.03) = 0.0309
　　　Positive posttest odds = 0.0309 × 7.4 = 0.229
　　　Positive posttest probability = 0.229 / (0.229 + 1) = 0.186 or 18.6%
　　　
　　　Thus, that individual has a post-test probability (or "post-test risk") of 18.6% of having bowel
　　　cancer.
　　　
　　　The prevalence in the population sample is calculated to be:
　　　Prevalence = (2 + 1) / 203 = 0.0148 or 1.48%
　　　The individual's pre-test probability was more than twice the one of the population sample,
　　　although the individual's post-test probability was less than twice the one of the population
　　　sample (which is estimated by the positive predictive value of the test of 10%), opposite to
　　　what would result by a less accurate method of simply multiplying relative risks.
　　　　

• 結論：運用多種診斷工具，以獲得有效的診斷力度。

參考資料

Wikipedia(2018, July, 28). Pre- and post-test probability: Revision history. Retrieved
      from http://t.cn/EGdrDWD
Bent, S., Gensler, L. S., Frances, C.(2007). Saint-Frances Guide: Clinical Clerkship in 
      Outpatient Medicine (Second Edition). Philadelphia City, United States, : Lippincott Williams &
      Wilkins.