Epidemiology

Both terms apply to using a test (or tests) to differentiate two clinical conditions
=> how good is this test in this situation??
in general, we can say FP and FN are FALSE, i.e. if there are a lot of FP and/or FN, this test is not good!
false positive means the positive test result while the truth is negative
(true positive means the positive test result while the truth is positive, i.e. FP + TP = all positive results)
e.g. Prostate specific antigen (PSA) is a tumor marker
lets take 4.0 as a cut off, i.e. if PSA <4.0, it is not prostate cancer; if PSA >= 4.0, it is cancer
now we have this case with PSA = 6.1, so by this number, the test result points to cancer
but this patient actually does not have cancer
So, this is a FALSE POSITIVE result
(of course now we know there are a lot of false positive and false negative cases if we interpret PSA testing in this way
PSA can be high while it is only benign conditions e.g. hyperplasia or prostatis, i.e. these conditions cause false positive results)

false negative means the negative test result while the truth is positive
(true negative means the positive test result while the truth is negative, i.e. FN + TN = all negative results; FN + TN + FP + TP = all neg + all pos = all results = 100%)
e.g. a man has a stomachache for a long time, so he goes for an endoscopy exam and a biopsy was taken
the report came back - no malignancy
but actually this man has stomach cancer
So, this is a FALSE NEGATIVE result
(why? because a BIOPSY is just a small piece of tissue, if in case the site is not correct, only normal tissue is taken, but not the cancerous tissue)
OK?

significance?
1. no test is perfect; every test has its limitation
2. we always need better test; if one test has high FP and/or high FN, choose another one! if there is not better one, develop a new one! (you need money to do so - the cost implication)
3. the most important message: if you are the one using the test (i.e. you are the clinician), remember that when you order the test, the test result should be handled carefully - WHAT YOU SEE MAY NOT BE TRUE. Consider what the possibilities are if it is not true, always correlate with clinical findings. (The truth is, it is not as easy as it seems, many frontline doctors are making this mistake.) The golden rule is: treat the patient, not the result.

這些東西好難三言兩語講得清楚喎！

簡單來講，任何的傳染病要作快速檢測的話，一定會根據某一指標而決定。不過，有關指標是非只適用於某一病例，還是可能是多種病例都會出現呢？又例如，會不會可能有一些非常特殊的case出現呢？這些都是false positive或false negative的意思。要知道，不論是醫學、化學或生物學，很多時都不是100%準確，而是有一個範圍的可能性。最近我睇一本叫《Blink》的書，就有提及過當初醫院決定是否把一個可能會出現急性胸口痛的病人收症時要考慮的因素。最初，醫院盡可能為病人做各種測試，但這卻使病人滯留在醫院裡。後來發現只有三個測試是必須的，其他的所測試的內容都已被這三個測試所包含。不過，即使如此，仍然有可能有漏網之魚。這些漏網之魚，就是在做三個test之時出現了false negative囉。又例如，女仔好多時都會買驗孕棒睇下有無BB，但如果有個女仔發現驗孕棒話佢有左，點知過左無耐佢又M到，那就是一個false positive的case了。

唔知你明唔明我講乜，又或者要不要用英文講多一次。