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Specificity vs. Sensitivity

Discussion in 'Medical Students - MD' started by Mike59, May 27, 2004.

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  1. Mike59

    Mike59 Sweatshop FP in Ontario 7+ Year Member

    Nov 19, 2001
    The inner-net
    Can someone please break down what these actually mean? The fancy definitions confuse me and I don't quite get how these are useful when reading over approaches to some conditions.
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  3. Seaglass

    Seaglass Quantum Member 10+ Year Member

    Jul 25, 2003
    Winston Salem, NC
    Specificity - the percent of time the test correctly identifies a condition.
    Sensitivity - the percent of conditions a test identifies.

    So, a test for prostate cancer with a sensitivity of 100% will never miss prostate cancer, but will likely identify a bunch of people as "positive" for prostate cancer who do not actually have the disease. (false positives)

    A test for prostate cancer with 100% specificity will never identify someone as having prostate cancer who does not. The downside is that a bunch of people with prostate cancer will be missed.

    A good example (continuing with the prostate) is PSA. I'm using hypothetical numbers here Kalel so don't shoot them down - I know they're not right.

    Generally, a PSA > 4 has been the action point for PSA. At this point the specificity is relatively low and the sensitivity is relatively high - most people (but not all) with prostate cancer will be identified at this point (sensitivity) but a number of them won't have prostate problems at all (specificity).

    If we dropped it to >1, we would catch almost all the cancer (very high sensitivity), but many more of those men would not actually have cancer (very low specificity). If we raised it to >9 almost certainly those men would have cancer (very high specificity) but we would miss many cases of cancer (very low sensitivity).

    It is uncommon to find a test that has both a high specificity and sensitivity. Usually it is a trade off. That is why often a combination of tests are used to evaluate for a particular pathology.

  4. exmike

    exmike NOR * CAL 10+ Year Member

    May 19, 2003
    Bay Area
    Sensitivity - how many of the true positives are actually detected? This is important for things such as HIV tests where you dont want to miss anyone. The more sensitive it is, the more of those infected are detected.

    Specificity - You can imagine it as the reverse. How many of the true negatives are detected? The more specific it is, the greater % of negative people will test negative.

    Usually increasing one will decrease the other.
  5. lattimer13

    lattimer13 good boy! 10+ Year Member

    Nov 11, 2002
    Pacific NW

    sensitivity= (#true positives)/(#true positives + #false negatives)

    specificity= (#true negatives)/(#true negatives + #false positives)
  6. babinski bob

    babinski bob Senior Member 7+ Year Member

    May 23, 2003
    Here's how I look at it...

    Sensitivity: for people w/ the disease, the liklihood of getting a positive test result.

    Specificity: for people w/o the disease, the liklihood of getting a negative test result.

    If you were designing a screening test, you'd want it to be as highly sensitive as possible, even if that meant picking up some false negatives.
    For a confirmatory test (after you've identified all the positives by your highly sensitive test), you'd want it to be as highly specific as possible.

    ***Remember, sensitivity rules OUT, specificity rules IN.

    For predicitive values...

    Positive predictive value: for people w/ a positive test result, the liklihood of having the disease.

    Negative predictive value: for people w/ a negative test result, the liklihood of not having the disease.

    Predictive values are unique in that they depend on prevalence (the % of population w/ a dz at a given 'snapshot' in time). As prevalence rises, your PPV rises, and NPV decreases. That's because if a disease is highly prevalent, there's a good chance that a positive test result means that person has the dz!
  7. AlexRusso

    AlexRusso Senior Member 5+ Year Member

    Apr 22, 2004
    I find the SpIN and SnOUT mneumonic very useful:

    SpIN- Specificity rules Positives In - meaning that on a test w. high specificity if you test positive chances are you got the disease (confirmatory)

    SnOUT - Sencitivity rules Negatives Out - meaning that on a test with high sencitivity if you test negative chances are you don't have the disease (screening)

    I also just memorized the 2x2 and that helps a lot as well

    Te-.... + TP....FP...PPV
    st...... - FN....TN...NPV

    TP=True Positive FP=False Positive FN=False Neg TN=True Neg PPV=Pos Pred Val NPV=Neg Pred Val

    To figure each you take the True value and divide by the total for tha row or column

    You can convince yourself then that if Sn is high that means TP must be high comapred to FN and you dont get a lot of FN hence if you're negative you prob don't have the disease (SnOUT)...you can do the same for the others

    I think you can solve most stats probs with that table...its MONEY
  8. med_stud_07

    med_stud_07 Member 5+ Year Member

    May 21, 2004
    Also, recall that spec. and sens. will always depend upon the prevalence in the population. Very important!!!! Depends on prevalence, yo!
  9. babinski bob

    babinski bob Senior Member 7+ Year Member

    May 23, 2003

    This is incorrect.... Predictive Values depend on prevalence.... Sensitivity and specificity do NOT!...

    See p 116 of First Aid: "Unlike sensitivity and specificity, predictive values are dependent on the prevalence of the disease. The higher the prevalence of a disease, the higher the positive predictive value of a test."
  10. med_stud_07

    med_stud_07 Member 5+ Year Member

    May 21, 2004
    Actually, what I meant was they calculate the sens. and spec. from the prevalence. The prevalence is used to determine these numbers. Thus, sens. and spec. DO depend on prevalence.

    I may be wrong, I frequently am. Let's examine the facts.
  11. AlexRusso

    AlexRusso Senior Member 5+ Year Member

    Apr 22, 2004
    As far as I know senc. and spec. DO NOT depend on prevalence. PPv and NPV do.
  12. Kev (UK)

    Kev (UK) British Member 7+ Year Member

    Jul 22, 2003
    This is correct!

    Sensitivity and Specificity will have the same predictive values in whatever population they are undertaken in. Positive Predictive Values (PPV) and Negative Predictive Values (NPV) will vary depending on the incidence of the disease or condition in that population. Foe instance the PPV for Chlamydia in 18-20 year old females will be higher than the PPV for Chlamydia in thirty-32 year olds simply because the incidence is higher in the younger age range.

    I remember Sensitivity as "the number of people who test positive who actually have the condition" and Specificity as "the number of people who test negative who do not have the condition".
  13. lotanna

    lotanna Child of God 7+ Year Member

    Oct 16, 2002
    Global nomad
    Epidemiology rocks!!! :D
    At least i know my epi degree will earn me some points on behavorial science :)

    Specificity and sensitivity dont depend on prevalence

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