Sensitivity, Specificity, Screening Tests & Confirmatory Tests
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- Опубліковано 25 чер 2024
- www.stomponstep1.com/sensitivi...
Sensitivity (Sen) & Specificity (Spec) are used to evaluate the validity of laboratory tests (not results of the tests). Basically, you use sensitivity and specificity to determine whether or not to use a certain test or to determine what situations a certain test would work best in. It is important to note that Sen and Spec are fixed for a certain test as long as you don't change the cutoff point. Therefore, Sensitivity & Specificity are not affected by changing prevalence. Both are given as a percentage ranging from 0% to 100%.
Sensitivity is the percentage of patients with the disease that receive a positive result or the percentage chance that the test will correctly identify a person who actually has the disease
Sensitivity = TP _ (TP + FN)
or
Sensitivity = TP _ Diseased
Specificity is the Percentage of patients without the disease that receive a negative result
Specificity = TN _ (TN+FP)
or
Specificity = TN _ Not Diseased
Imagine you have 2 very different guns. The first gun fires when you barely touch the trigger. A strong gust of wind could set it off. The first gun has high sensitivity and low specificity. It is sensitive to the smallest of signals to fire while not being very specific to an intentional pull of the trigger. You never miss a possible chance to shoot your gun (~ Low FN), but you often accidentally fire when you shouldn't (~ High FP). The second gun only fires if you pull the trigger really hard. This gun has high specificity and low sensitivity. It is very specific to firing only when you intentionally pull the trigger (~Low FP), but it isn't very sensitive to a weak pull of the trigger (~High FN).
In the real world you never have a test that is 100% Sen and 100% Spec. We are usually faced with a decision to use a test with high Sen (and lower spec) or high Spec (and lower Sen). Usually a test with high sensitivity is used as the Initial Screening Test. Those that receive a positive result on the first test will be given a second test with high specificity that is used as the Confirmatory Test. In these situations you need both tests to be positive to get a definitive diagnosis. Getting a single positive reading is not enough for a diagnosis as the individual tests have either a high chance of FP or a high chance of FN. For example, HIV is diagnosed using 2 tests. First an ELISA screening test is used and then a confirmatory Western Blot is used if the first test is positive.
There are also specific situations where having a high specificity or sensitivity is really important. Consider that you are trying to screen donations to a blood bank for blood borne pathogens. In this situation you want a super high sensitivity, because the drawbacks of a false negative (spreading disease to a recipient) are way higher than the drawbacks of a false positive (throwing away 1 blood donation). Now consider you are testing a patient for the presence of a disease. This particular disease is treatable, but the treatment has very serious side effects. In this case you want a test that has high specificity, because there are major drawbacks to a false positive.
Now that you have finished this video you should check out the next video in the Biostats & Epidemiology section which covers the calculation of Predictive Value Positive & Negative (www.stomponstep1.com/negative-.... That video has some mnemonics and concepts that also apply to this video.
I can't thank you enough for making all these videos. Straight to the point and well explained. I hope you become a very successful physician and a happy life.
Thank you! Can't ever remember the difference between the two and your gun analogy really helped and the C for confirmatory and S for screening solidified understanding of clinical application. (Taking my APRN boards this week)
Happy I stumbled upon this, very helpful :-)
Please continue your work. It is very straightforward and easy to understand. Thank you (I'm in Biostatistics/Epidemiology for Master's in Nursing)
Thank you!
i have no money .......so i am very happy u have topics that i can understand better fo step one
i watched ur hypersensitivity video and i thank god for u. im in nursing school and it helped me understand it so much best ever video. will u ever post more videos on other disease processes?
+jade m Thank you so much for the kind words :) I plan on posting videos covering pretty much everything I learned in medical school, but it will unfortunately take me a LONG time to accomplish that goal. I'm still a full time medical student (and part time masters in public health student) so I don't have a ton of free time and each videos usually takes dozens of hours of work from start to finish. But keep an eye out for more videos as I hope to release a few videos every month or so.
really helpful. i never understand spin in spin out.
S=screen C=confirm is the best way to remember.
also last part of the example is really useful for application when to select......Thanks
thank
Thanks ..very useful
+pradeeprox87 thanks for commenting!
Validity of diagnostic test
I thought ROC curves was high yield??
MegaKaylicious Understanding the ROC curve may help you understand other highyield topics but I think the odds of you being given an ROC curve on your Step
1 exam is very low. My high yield rating is mostly based on the frequency of occurrence
in a giant sample of practice questions (>5000) that includes commercially
available practice questions and retired Step 1 questions (AKA the NBME Self Assessments).
Out of that huge sample of questions there is not a single ROC curve and I didn’t
have one when I took the real exam. You can learn more about my high yield rating
here www.stomponstep1.com/high-yield-rating-hyr/ That being said ROC curves are listed on the USMLE content outline so it is still possible you will get one
Stomp On Step 1 hey do you have a list of topics that you rated high yield?? ... great videos by the way !!
Abraham Cho Here is the biostats list www.stomponstep1.com/high-yield-rating-list-for-biostatistics-and-epidemiology/ & here is Biochem www.stomponstep1.com/usmle-biochemistry-high-yield-rating-biochem-for-step-1/ I unfortunately haven't had a chance to make a list for the other chapters yet. Hope to have those soon
Abraham Cho And I haven't made a video for it yet but here is the Genetics list
10 - Autosomal Recessive Inheritance
9 - X Linked Recessive Inheritance
9 - Pedigrees
7 - Post translational Modification
6 - Translation
6 - Trinucleotide Repeat Expansion
5 - Mitochondrial Inheritance
5 - Tuberous Sclerosis
4 - Epigenetics
3 - DNA Replication
3 - Imprinting, Angleman & Prder-Willi
3 - Polygenic Inheritance
2 - Variable Expressivity
4 - Autosomal Inheritance
4 - Lesch Nyhan
3 - Neurofibromatosis
3 - Down Syndrome
2 - Variable Expressivity
2- Mosaicism
2 - Pleiotrophy
2 - Incomplete Penetrance
2 - Von Hippel Lindau
No Yield:
• Nucleotide & DNA/RNA Structure
• Ligase, Telomerase, Helicase, Primase ….
• Okazaki Fragments & the process of transcription
• DNA Repair Mechanisms & Related Diseases
• Gene Regulation
• Specifics about Protein Synthesis
• Meiosis & Mitosis
• X-Linked Dominant
• Hardy-Weinberg Calculations
• Uniparental disomy
• Locus heterogeneity
• Pyrimidine Synthesis
• Purine Synthesis
• Orotic Aciduria
• Sturge Weber Syndrome
• Osler-Weber-Rendu Syndrome
• Ataxia Telangiectasia
• Cri du Chat
• Edwards
• Patau
• Friedrichs Ataxia
• Myotonic dystrophy
wow this is amazing !! thanx so much... your videos are great ...going to finish up the biostats tonight
forshee pious
You got the equation wrong: sensitivity= TP/(TP+FP)!, but really good otherwise
no he didn't bro
Despicable mo No, you are wrong