tag:blogger.com,1999:blog-4956989639073843954.post8949303583236944506..comments2024-03-21T05:30:03.220-04:00Comments on Science teacher: One problem with drug screeningdoylehttp://www.blogger.com/profile/12901661320505882735noreply@blogger.comBlogger8125tag:blogger.com,1999:blog-4956989639073843954.post-75860380793265973692008-12-02T18:33:00.000-05:002008-12-02T18:33:00.000-05:00@DoyleThanks for looking into it. I always new th...@Doyle<BR/>Thanks for looking into it. I always new the Ann Arbor police were pretty hard nosed. The counseling follow up is really the cornerstone of the program - ideally preventing minor problems from turning into dependence issues or worse. <BR/><BR/>Our diocese chose this company due to their " highly scientific" approach - whatever that means. Our parents can always appeal the test for further follow up so I suppose that would eliminate the false negatives but I'd love to be sure. <BR/><BR/>email is charlieroy1977@gmail.comCharlie Royhttps://www.blogger.com/profile/09335346223868916197noreply@blogger.comtag:blogger.com,1999:blog-4956989639073843954.post-64985641788107004552008-12-01T19:43:00.000-05:002008-12-01T19:43:00.000-05:00As unseemly as it is to post multiple messages on ...As unseemly as it is to post multiple messages on one's blog, I cannot find your email, Souly, so an update here.<BR/><BR/>The link gets me to the testing company, but not the video--there is no mention of sensitivity nor specificity except in vague terms.<BR/><BR/>I've been googling around to see what I can find, but so far I've only managed to cough up a couple of articles, both of which require me to pay a fee--and I'm cheap.<BR/><BR/>I'll get back to you when I learn more.doylehttps://www.blogger.com/profile/12901661320505882735noreply@blogger.comtag:blogger.com,1999:blog-4956989639073843954.post-86035175844795305792008-12-01T17:49:00.000-05:002008-12-01T17:49:00.000-05:00Of course, Ken, I just screwed up my own argument ...Of course, Ken, I just screwed up my own argument by using an example that shows less than half are false positives, but I hope the larger point was made.<BR/><BR/>Cheers!doylehttps://www.blogger.com/profile/12901661320505882735noreply@blogger.comtag:blogger.com,1999:blog-4956989639073843954.post-44160724492312017582008-12-01T17:47:00.000-05:002008-12-01T17:47:00.000-05:00Dear Souly,I don't know what the specificity and t...Dear Souly,<BR/><BR/>I don't know what the specificity and the sensitivity of a particular hair screening test are (I will look at the video when I get a chance tonight), but the false positive rate depends on the prevalence of drug use in your population. The lower the prevalence, the higher the false positive rate.<BR/><BR/>I agree that if the presence of the test gives people an excuse to say no, it may have some value. I don't know of any studies that show this, but it makes sense. Ironically, one of the nice things about being a college student in Ann Arbor was that since marijuana was essentially legal (a $5 fine), there was no pressure to smoke a joint just to prove you are not a narc.<BR/><BR/>If you have a concern about the screening test, you can follow it up with tests with higher specificity at more cost. Screening tests are just that--meant to screen. I applaud your counseling program; I think any screening program requires both follow-up testing and a program designed to help fix the problem.<BR/><BR/>Kia ora Ken<BR/><BR/>I may be debating a different point. My fear is that those in charge of testing do not grasp what a positive test means in a population that does not generally use the drugs being screened.<BR/><BR/>You are right--the specificity does not change, but the positive predictive value (the chance that a person who tests positive is actually using drugs) does. In a population with a very low incidence of drug use, most of the positive tests can, in fact, be false positives.<BR/><BR/>If a test is 99% specific (and assume it's 100% sensitive), and 2% of the population is using the drugs beings screened, we get the following results if we test 1000 students.<BR/><BR/>The 20 actually using the drugs will test positive. We have 980 left. Of those 980, about 10 will test positive. That means the positive tests will be wrong about a third of the time.<BR/><BR/>If 50% of the population is using the screened drugs, then 500 will test true positive, and about 5 of the remaining 500 will test false positive. The positive tests then are accurate 99% of the time.<BR/><BR/>I used to argue with fellow docs chasing their own tails all the time--they'd use a shotgun approach to diagnosis, order a million tests, then get all excited when one of the results came back positive even if the clinical history did not fit the result. They'd argue "95% sensitive, 95% specific, patient has a 95% chance of having the disease"--and another patient goes down the garden path of pointless fear and testing.<BR/><BR/>Now if we could come up with a cheap screening test that's 100% specific and 100% sensitive, no worries.<BR/><BR/>(Screening tests are, by their nature, meant to have high sensitivities at low cost of testing--positive screening tests should be followed up with more accurate, and usually more expensive, tests.)doylehttps://www.blogger.com/profile/12901661320505882735noreply@blogger.comtag:blogger.com,1999:blog-4956989639073843954.post-21816642519777718462008-12-01T15:22:00.000-05:002008-12-01T15:22:00.000-05:00Kia ora MichaelI don't doubt that the incidence of...Kia ora Michael<BR/><BR/>I don't doubt that the incidence of false positives is too high. That wasn't my argument at all.<BR/><BR/>In 10,000 clean people tested you will on average get 10 false positives, if the specificity is 99.9%, no matter HOW you choose the sample of 10,000. This is my point.<BR/><BR/>It should not be based on the number of actual pot heads in the sample at all. BUT. . . <BR/><BR/>You are right, 10 false positives is 10 too many, whichever way you look at it, for it does not justify the use of the test, especially if the incidence of actual pot heads is of the same order or less.<BR/><BR/>Frankly, I don't think ANY argument justifies the use of a testing system that returns that percentage of false positives from a clean sample.<BR/><BR/>Catchya laterBlogger In Middle-earthhttps://www.blogger.com/profile/08722634477041121797noreply@blogger.comtag:blogger.com,1999:blog-4956989639073843954.post-50590231971895400102008-12-01T06:27:00.000-05:002008-12-01T06:27:00.000-05:00I am the principal of a school that has mandatory ...I am the principal of a school that has mandatory drug testing. I find the conversations about false negatives to be very thought provoking. We use a hair test that gives a 90% drug history. We've had the program now for ten years. On average less than 1% of our students test positive each year. In general the students on annual surveys indicate the fact that they will be tested at school gives them a reason to say "no". <BR/><BR/>What I'm curious about is whether this hair test method as fell contains false negative. I don't want to discipline a student who doesn't deserve it. Although discipline is a tough thought considering our policies for a positive call for mandatory family and individual counseling - a more reform based model. <BR/><BR/>Take a look at their video http://www.drugtestwithhair.com/# . I'd appreciate some input from those more science-savy than me.Charlie Royhttps://www.blogger.com/profile/09335346223868916197noreply@blogger.comtag:blogger.com,1999:blog-4956989639073843954.post-85012024820350292882008-12-01T05:53:00.000-05:002008-12-01T05:53:00.000-05:00Kia ora KenIt's not a false logic--the prevalence ...Kia ora Ken<BR/><BR/>It's not a false logic--the prevalence of the condition you are looking for (in this case, specific drug use) determines the likelihood that a positive result means that a randomly selected person in that population is true positive.<BR/><BR/>This is why physicians take histories before doing tests. This is why the CDC came down hard on docs who used HIV screening tests without getting histories first.<BR/><BR/>You are right that if all were using pot, there would be no false positives; there would also be no need to do the test.<BR/><BR/>(In schools where half or more of the kids are getting drunk on weekends, it's just as pointless to screen.)<BR/><BR/>As an aside, a 100% sensitive test is easy--here's one. I will draw your blood and put a drop on the table--if it's red, you're positive. Bingo, 100% sensitivity (though useless since there's no specificity).<BR/><BR/>The problem is that the percent of false positives gets unacceptably high (we both love puns it seems) if you screen a population with low drug use. The bigger problem is that most people administering the test do not realize this. <BR/><BR/>You're right that the absolute number of monks testing positive on the island would drop if the prevalence drops--but the false positive rate rises dramatically.<BR/><BR/>Larger sample sizes won't change the percentages. <BR/><BR/>Even with a specificity of 99.9%, a positive test means little if the prevalence of drug use is low.<BR/><BR/>I'm not worried about the users here (though I do worry about them for other reasons); I worry about those falsely accused. Here in the States many employers require a screening test. A false positive can travel a long way in the information world.doylehttps://www.blogger.com/profile/12901661320505882735noreply@blogger.comtag:blogger.com,1999:blog-4956989639073843954.post-31587952747455177572008-12-01T05:16:00.000-05:002008-12-01T05:16:00.000-05:00Kia ora MichaelForget about your 99.9% specific re...Kia ora Michael<BR/><BR/>Forget about your 99.9% specific red herring – you didn’t do a comparison with that specificity.<BR/><BR/>You are calculating the falseness of the test with reference to the total number found positive. This is where your logic goes all to pot.<BR/><BR/>If they were all on the pot there would be absolutely no false tests returned. Right?<BR/><BR/>The number of false tests will naturally decrease the greater the proportion of crack heads in the sample. It comes down to proportion and the false logic of always calculating your percentage on the number left who are straight.<BR/><BR/>It’s to do with your sampling for your calculation. When the proportion of crack heads increases, you really should adjust your sample size to be fair. This is because, as you say, the test is 100% sensitive.<BR/><BR/>So you should base your calculation on the same number of pot-frees every time. It’s got nothing to do with the number of crack heads, for we know they are always positive anyway.<BR/><BR/>Catchya later<BR/>from Middle-earthBlogger In Middle-earthhttps://www.blogger.com/profile/08722634477041121797noreply@blogger.com