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Choosing and Using Diagnostic Tests
The EBM strategy for integrating clinical
differential diagnosis and test choice is illustrated in an
interesting article on how to use an article about Disease
Probability for Differential Diagnosis, published in JAMA
in 1999 [1].
Fundamentally, the theory is that a differential
diagnosis can usually be constructed and integrated with investigation
and management as follows:
| DIFFERENTIAL
|
TEST
CHOICE |
TREATMENT |
| Leading
Hypothesis |
High
specificity to confirm |
Start
Tx |
| Active
alternatives |
High
sensitivity to exclude |
Not
routinely |
| Others |
No
testing |
No
treatment |
| Excluded
alternatives |
No
testing |
No
treatment |
The implication for radiologists is that,
for optimal consultations and protocols, it is important for
us to discriminate between the strength of an imaging test
when it is abnormal and when it is normal. Having done this,
if we aim to identify the tests / imaging signs with >
95% sensitivity or specificity for particular clinical indications
we should be able to 'rule in’ or ‘rule out’
differential hypotheses with ease. We should, where possible,
avoid relying on weaker tests / imaging signs.
Useful mnemonics [2]
are:
SpPIN: When a test of high Specificity
is Positive, the disease in question is ruled IN
SnNOUT: When a test of high Sensitivity
is Negative, the disease in question is ruled OUT.
References.
1. Richardson WS,
Wilson MC, Guyatt GH, Cook DJ ,Nishikawa J, Users' guides
to the medical literature: XV. How to use an article about
disease probability for differential diagnosis. Evidence-Based
Medicine Working Group. Jama 1999; 281 (13):1214-1219. [ link
]
2. Sackett DL, Strauss
SE, Richardson WS, Rosenberg W ,Haynes RB, Diagnosis and Screening,
in Evidence Based Medicine; How to Practice and Teach EBM.
2000, Churchill Livingstone: Edinburgh. p. 67-93. [ link
]
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