Rapid Flu Test Usefulness --> RE: Flu and Tamiflu Resistance

[Guest guest]

I won't keep blasing everyone with this info, but some have pointed out the false (+) rates on rapid flu tests are high -- esp when prevalence is very low.

Some info below.

Interestingly, based on the data below...

My lab specificity for the rapid flu test done is pretty Good, but the prevalence of flu is low -- so I would be expected to get false +'s of 44-61% currently -- interesting.

If Flu Prevalence is…

And Specificity is…

Then PPV is…

False Pos. rate is…

VERY LOW (2.5%)

POOR (80%)

V. POOR (6-12%)

V. HIGH (88-94%)

VERY LOW (2.5%)

GOOD (98%)

POOR (39-56%)

HIGH (44-61%)

Our lab uses the test below...

http://www.genzymediagnostics.com/pdf/OSOM_Influenza_190_PI.pdf

http://www.cdc.gov/flu/professionals/diagnosis/rapidlab.htm

The rapid tests vary in terms of sensitivity and specificity when compared with viral culture. Product insert information and research publications indicate that:

Median sensitivities are approximately 70-75% Median specificities are approximately 90-95%

Accuracy Depends Upon Prevalence

The positive and negative predictive values vary considerably depending upon the prevalence of influenza in the community.

False-positive (and true-negative) influenza test results are more likely to occur when disease prevalence is low, which is generally at the beginning and end of the influenza season. False-negative (and true-positive) influenza test results are more likely to occur when disease prevalence is high, which is typically at the height of the influenza season.

Clinical Considerations of Testing When Influenza Prevalence is Low

When disease prevalence is relatively low, the positive predictive value (PPV) is low and false-positive test results are more likely. By contrast, when disease prevalence is low, the negative predictive value (NPV) is high, and negative results are more likely to be true.

If Flu Prevalence is…

And Specificity is…

Then PPV is…

False Pos. rate is…

VERY LOW (2.5%)

POOR (80%)

V. POOR (6-12%)

V. HIGH (88-94%)

VERY LOW (2.5%)

GOOD (98%)

POOR (39-56%)

HIGH (44-61%)

MODERATE (20%)

POOR (80%)

POOR (38-56%)

HIGH (44-62%)

MODERATE (20%)

GOOD (98%)

GOOD (86-93%)

LOW (7-14%)

The interpretation of positive results should take into account the clinical characteristics of the case. If an important clinical decision is affected by the test result, the rapid test result should be confirmed by another test, such as viral culture or polymerase chain reaction (PCR).

Clinical Considerations of Testing When Influenza Prevalence Is High

When disease prevalence is relatively high, the NPV is low and false-negative test results are more likely. When disease prevalence is high, the PPV is high and positive results are more likely to be true.

If Flu Prevalence is…

And Sensitivity is…

Then NPV is…

False Neg. rate is…

MODERATE (20%)

POOR (50%)

MODERATE (86-89%)

MODERATE (11-14%)

MODERATE (20%)

HIGH (90%)

V. GOOD (97-99%)

V. LOW (2-3%)

HIGH (40%)

POOR (50%)

MODERATE (70-75%)

MODERATE (25-30%)

HIGH (40%)

HIGH (90%)

V. GOOD (93-94%)

LOW (6-7%)

The interpretation of negative results should take into account the clinical characteristics of the patient. If an important clinical decision is affected by the test result, then the rapid test result should be confirmed by another test, such as viral culture or PCR.

Locke, MD