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The time-efficiency principle: time as the key diagnostic strategy in primary care.

Irving G, Holden J - Fam Pract (2013)

Bottom Line: This approach safely and efficiently reduces the number of patients who need to be formally tested in order to make a correct diagnosis for a person.Time, in terms of observed disease trajectory, provides a vital mechanism for achieving this task.It remains the best strategy for delivering near-optimal diagnoses in low-prevalence settings and should be used to its full advantage.

View Article: PubMed Central - PubMed

Affiliation: Department of Primary Care, University of Liverpool, Liverpool, UK. greg.irving@liv.ac.uk

ABSTRACT
The test and retest opportunity afforded by reviewing a patient over time substantially increases the total gain in certainty when making a diagnosis in low-prevalence settings (the time-efficiency principle). This approach safely and efficiently reduces the number of patients who need to be formally tested in order to make a correct diagnosis for a person. Time, in terms of observed disease trajectory, provides a vital mechanism for achieving this task. It remains the best strategy for delivering near-optimal diagnoses in low-prevalence settings and should be used to its full advantage.

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The effect of prevalence on number needed to predict for an excellent sign, symptom or laboratory test. Figure constructed using data presented by Sackett et al.6
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Figure 2: The effect of prevalence on number needed to predict for an excellent sign, symptom or laboratory test. Figure constructed using data presented by Sackett et al.6

Mentions: The principal justification for performing a diagnostic test is to gain new information. Test results can have little impact on clinical decision-making when the prevalence of disease is at extremes because there is little difference between the pre- and post-test probabilities.6 For any test, the relationship between prevalence and predictive values is dynamic.10 The total gain in certainty, as indicated by the predictive summary index (PSI, ψ = NPV + PPV − 1) is greatest when the pre-test probability of the disease (prevalence) is around 50% (Table 1, Fig. 1).6,7 Similarly, the number of persons needed to be examined in order to correctly predict a diagnosis of the disease (number needed to predict, NNP = 1/ψ) is also at its lowest around this midpoint (Table 1, Fig. 2). Here, a positive result from a sensitive and specific test almost guarantees the diagnosis and a negative result effectively eliminates the target disorder(s) from the differential diagnosis. All tests are of their greatest diagnostic use in the 50:50 dilemma when the pre-test probability of the target disease is equally likely to be present or absent. However, in practice, the time-efficiency principle will operate most effectively at low prevalences, typically between 0% and 10% (Fig. 2). Here, each small increase in prevalence results in the greatest fall in NNP, i.e. tests at follow-up rapidly become more useful. This is helpful as the probable prevalence of most major pathologies presenting in primary care is <10%. The diagnostic task is to then apply more refined tests, such as imaging or specialist opinion, for patients who have reached the point of flattening on the parabola, around the 10% prevalence point. It is the particular skill of a GP working in a low-prevalence setting to safely reach this 10% zone, where definitive testing or referral becomes much more useful, by using the time-efficiency principle.


The time-efficiency principle: time as the key diagnostic strategy in primary care.

Irving G, Holden J - Fam Pract (2013)

The effect of prevalence on number needed to predict for an excellent sign, symptom or laboratory test. Figure constructed using data presented by Sackett et al.6
© Copyright Policy - creative-commons
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3722505&req=5

Figure 2: The effect of prevalence on number needed to predict for an excellent sign, symptom or laboratory test. Figure constructed using data presented by Sackett et al.6
Mentions: The principal justification for performing a diagnostic test is to gain new information. Test results can have little impact on clinical decision-making when the prevalence of disease is at extremes because there is little difference between the pre- and post-test probabilities.6 For any test, the relationship between prevalence and predictive values is dynamic.10 The total gain in certainty, as indicated by the predictive summary index (PSI, ψ = NPV + PPV − 1) is greatest when the pre-test probability of the disease (prevalence) is around 50% (Table 1, Fig. 1).6,7 Similarly, the number of persons needed to be examined in order to correctly predict a diagnosis of the disease (number needed to predict, NNP = 1/ψ) is also at its lowest around this midpoint (Table 1, Fig. 2). Here, a positive result from a sensitive and specific test almost guarantees the diagnosis and a negative result effectively eliminates the target disorder(s) from the differential diagnosis. All tests are of their greatest diagnostic use in the 50:50 dilemma when the pre-test probability of the target disease is equally likely to be present or absent. However, in practice, the time-efficiency principle will operate most effectively at low prevalences, typically between 0% and 10% (Fig. 2). Here, each small increase in prevalence results in the greatest fall in NNP, i.e. tests at follow-up rapidly become more useful. This is helpful as the probable prevalence of most major pathologies presenting in primary care is <10%. The diagnostic task is to then apply more refined tests, such as imaging or specialist opinion, for patients who have reached the point of flattening on the parabola, around the 10% prevalence point. It is the particular skill of a GP working in a low-prevalence setting to safely reach this 10% zone, where definitive testing or referral becomes much more useful, by using the time-efficiency principle.

Bottom Line: This approach safely and efficiently reduces the number of patients who need to be formally tested in order to make a correct diagnosis for a person.Time, in terms of observed disease trajectory, provides a vital mechanism for achieving this task.It remains the best strategy for delivering near-optimal diagnoses in low-prevalence settings and should be used to its full advantage.

View Article: PubMed Central - PubMed

Affiliation: Department of Primary Care, University of Liverpool, Liverpool, UK. greg.irving@liv.ac.uk

ABSTRACT
The test and retest opportunity afforded by reviewing a patient over time substantially increases the total gain in certainty when making a diagnosis in low-prevalence settings (the time-efficiency principle). This approach safely and efficiently reduces the number of patients who need to be formally tested in order to make a correct diagnosis for a person. Time, in terms of observed disease trajectory, provides a vital mechanism for achieving this task. It remains the best strategy for delivering near-optimal diagnoses in low-prevalence settings and should be used to its full advantage.

Show MeSH