Diagnostic tests Flashcards
Define sensitivity in relation to diagnostic test results
The proportion of true positive identified by the test as having the disease i.e. the ability of the test to identify individuals with the disease
What is the practical significance of high sensitivity?
- Minmise false negatives, more likely to pick up false positives (but will pick up every positive)
- Therefore make good screening tests
- Rule out disease (SnOut)
Define specificity in relation to diagnostic test results
The proportion of true negatives identified by the test as not having the disease i.e. indicates the ability of the test to identify individuals without a disease
What is the practical significance of high specificity?
- Will only pick up positives if are truly positive, minimises false positives
- Will pick up every negative (and maybe some false negative results)
- Therefore high spec is good for confirmation, rule in disease (SpIn)
what is the relationship between sensitivity and specificity?
As one increases, the other decreases
Define the positive predictive value
The number of true positives compared to the total number of positives given by a test (i.e. of 100 positive results, 88 are actually positive for the disease)
What does a high positive predictive value mean?
That the test is accurate for a positive result most of the time (of 100 positive results, 99 are trule positive so PPV = 99%)
Define negative predictive value
The number of true negatives compared to the total number of negatives
What does a high negative predictive value mean?
Can be confident in teh negative values most of the time i.e. of 100 negative results, 99 were truly negative if NPV is 99%
What is the prevalence of a disease?
The number of diseased cases within a population
What is the effect of sample size on prevalence?
- Large sample size with little discrimination in terms of patient’s tested will have lower prevalence
- Small sample size with strict discrimination in terms of patient’s tested for disease will have higher prevalence as the test will more often give a positive result
What is the effect of prevalence on positive and negative predictive value?
Lower prevalence means less reliable for a positive result i.e. lowered PPV (will get more negative results than positive ones)
Why might a single measurement of blood concentration of a hormone not reflect the true status of the gland?
- May fluctuate during the day
- May have short half life
- May be labile and not store very well
- Influenced by normal physiological stimuli e.g. stress, exercise
- Influenced by feeding
Why might a provocative or dynamic hormone test be useful?
- Removes effect or normal physiological fluctuations
- Can assess the response to stimulation or suppression and so differentiate between normal and diseased animals
- i.e. subnormal response in stimualtion may suggest deficiency
- Supraphysiological response to stimulation may suggest hormone excess/lack of feedback control, as would inability to suppress response
- may also help differentiate primary and secondary disease, localise lesion
What are the 2 broad categories of blood sampling tubes?
- Coagulation promotors
- ## Coagulation inhibitors
