Hi Kellie,
This is a challenging concept that still confuses me as well! Below is a posting from last year that seemed to help (not sure if you could see it so I copied it below).
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).
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.
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.