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February 20, 2020 at 9:17 pm #318901
Can you give guidance on sensitivity vs specificity and sPin vs sNout?
75. A standardized assessment that helps identify gross motor delay in children has a sensitivity of 98% and a specificity of 50%. After performing the assessment, the scores indicate that the child has a gross motor delay. How can you interpret this information?
a. The assessment has a high success rate of identifying gross motor delays but may also suggest that there is a gross motor delay when there is not really a delay present
b. The assessment has a high success rate of correctly identifying children who do and do not have a gross motor delay
c. The assessment only has a high success rate of correctly identifying children who do not have a gross motor delay
d. The assessment has a high success rate of identifying children who do not have gross motor delay but may miss some children who actually do have a gross delay
The sensitivity of a test is the percentage of individuals with a particular condition who are correctly identified as positive by the test. The specificity is the percentage of individuals without a particular condition who are correctly identified as negative by the test. Tests with high sensitivity often have low specificity. They are good for accurately identifying a condition but they also come with a fairly high rate of false positives.
February 22, 2020 at 5:54 pm #320333
These concepts have rattled me for years! a. is the correct answer for this question: “The assessment has a high success rate of identifying gross motor delays but may also suggest that there is a gross motor delay when there is not really a delay present.”
SNout: When a test has a high sensitivity, a negative result rules out the diagnosis and positive result confirms the diagnosis.
SPin: When a test has a high specificity, a positive result tends to rule in the diagnosis and negative result rules out condition.
(I actually find these very confusing, so trying looking at it from the examples below):
Sensitivity (true positive rate): Likelihood that someone with the condition will be positive on the diagnostic test; refers to the percentage of children who are correctly identified as meeting criteria for a condition; VALUABLE FOR CONFIRMING DX; 80% sensitivity is preferable.
Specificity (true negative rate): Likelihood that someone who does not have the condition will be negative on the diagnostic test; refers to the percentage of children without problems who are correctly identified as such; VALUABLE TO RULE OUT THE PRESENCE OF A CONDITION; 90% is preferable for a diagnostic test.
**For the TIMP, Specificity is higher, indicating that it is a good measure for detecting large numbers of high risk infants who are not developing typically (rule out GM delay) and have delayed posture and motor development, which is it’s intended purpose. Sensitivity is much lower, so it is not as sensitive for identifying (or confirming) CP.
**The AIMS has been found to be more beneficial when administered at critical age ranges. While the 5th centile cut off value is best to identify the most delayed children (and those likely to have CP) (because of high specificity and relatively high sensitivity), the 10th centile cut off is best for identification of the greatest # of infants with abnormal motor development (because of high sensitivity).
There is always a “trade-off” between sensitivity and specificity: for a screening test in which early diagnosis is beneficial and when it is desirable to identify all those at risk for having a condition, high sensitivity is preferable to higher specificity.
February 24, 2020 at 12:33 pm #321695
Thank you.. I have to read over it multiple times, I don’t know why these terms always seem to confuse me.
When you give the example of the TIMP – Specificity is higher –Likelihood that someone who does not have the condition will be negative on the diagnostic test; refers to the percentage of children without problems who are correctly identified as such; — so with the TIMP, when looking at Specificity, we are looking at the percentage of children without the the condition who are correctly negative on the test, and ruling out a condition — so we are ruling out GM delay with the TIMP Score —
But I am still confused by the explanation of TIMP: “indicating that it is a good measure for detecting large numbers of high risk infants who are not developing typically” — this seems like sensitivity – we are are identifying infants not developing typically (those who have GM Delay)?
Sorry, this topic as had me since PT school
February 24, 2020 at 7:41 pm #321956
Sensitivity focuses on a specific condition (the example I gave is CP) whereas specificity focuses on ruling out the presence of a condition (the example I gave is GM delay). A child with poor gross motor skills will very likely score below average on the TIMP and be correctly identified as being delayed, however, we won’t be able to necessarily determine if the low performance is due to CP or another condition affecting GM development. A test with high sensitivity is best when your focus is identifying CP, not just GM delay.
My brain wants to switch the concepts since I tend to think of “specificity” as identifying a specific condition (being “specific” about a condition), when it is actually the opposite.
Hope this helps!
February 24, 2020 at 8:28 pm #321964
That does help! i also had a lengthy discussion with my co-workers on the topic, which also helped. Thank you for the time you spend in helping understand this… less than 3 weeks to test day!
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