Positive NAAT test results for Neisseria gonorrhoeae do not require routine

confirmatory testing

Matthew R. Golden MD, MPH

Center for AIDS & STD, University of WA

Public Health - Seattle & King County

Objectives

• To demonstrate that NAAT for gonorrhea do not require universal confirmatory testing when used in low prevalence environments.

• Issues I will not argue

• Should low prevalence populations be screened

• Selective screening – currently do not exist

• PCR can be used in very low prevalence

populations without confirmatory testing

Problems with an argument based on PPV

• Routine confirmatory testing is impractical

• Need for routine testing reflects a hyperbolic sense of risk, and excessive attention to relative rather than absolute risk

• Perceived need is based on notion that true test performance in a very low prevalence population can’t be known. It’s cynical.

• Maybe some tests are OK?

Approaches to confirmatory testing

Approach Barriers to implementation

Barriers to interpretation

Overall

Test a new specimen +/- different test

Patient needs to return

Which test is correct? – helpful if both are positive

- Good for usual circumstances- Culture is definitive- Having patients return is not practical

Repeat test using a different test

-Incompatible media- Need for labs to maintain >1 test or send out – turn around time

Which test is correct? – helpful if positive

Best option, but will be tough to implement unless the same testing platform is used

Repeat same test Easy Two positives still hard to interpret.

Discrepant- Which test is correct?

Easiest, but least definitive

50

67

959284

9691

97999896

83

0

20

40

60

80

100

0% 5% 10% 15% 20%

99% Specificity99.5% Specificity99.8% Specificity

Pos

itive

Pre

dict

ive

Val

ue (

%)

Prevalence

Positive predictive value is primarily a function of prevalence & specificity

1%

The absolute risk & number of false positives varies little with prevalence

Prevalence

1%

4%

8%

# false positives per 10,000

98

95

91

PPV

49%

80%

89%

Assumes 95% sensitive test and 99% specific

Absolute risk false positive

0.98/100

0.95/100

0.91/100

Are NAAT tests good enough?

• How good do they need to be?

• PPV >90% in a very low prevalence

population (i.e. 0.5%)

Size, Specificity and Prevalence of Larges NAAT Studies

Largest study Specificity in largest study

Lowest prevalence

studied

BDProbetec 3,544 99.3 3.6%

Digene HC-II 1,370 98.5 6.3%

Aptima 1,484 98.7 8.5%

Specificity estimates are imprecise.

Recent studies have avoided discrepant analysis, probably erring on the side of underestimating specificity

6,2078,08310,560

15,09423,396

40,71389,342

1

10

100

1,000

10,000

100,000

1,000,000

99.92 99.93 99.94 99.95 99.96 99.97 99.98

# te

sted

True Specificity (%)

Number of specimens needed to define PPV >90% in a 0.5% prevalent population

874

394

57

224

14399 75

6207808310560

1509423396

4071389342

1

10

100

1000

10000

100000

1000000

99.92 99.93 99.94 99.95 99.96 99.97 99.98

# te

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0

200

400

600

800

1000Confirmatory testing design

Cohort design

# t

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Specificity (%)

Number of specimens needed to define PPV >90% in 0.5% prevalent population

PPV Aptima for N. gonorrhoeae in a low prevalence population

• 0/194 negative specimens tested positive for N. gonorrhoeae

4 Labs in WA state test 59,664 specimens 280 (0.5%) GC+

265 Tested using alternative set rRNA primers

258 GC+PPV= 97.4 (95% CI 95.1%-98.8%)

Limitations to Confirmatory Testing

• Using a NAAT for confirmatory testing assumes that the confirmatory NAAT is specific. If an organism other than N. gonorrhoeae has nucleic acid amplification sequences amplified by both tests, then a confirmatory NAAT results will not be valid.

• Routine confirmatory testing doesn’t solve this problem unless all the patients come back for culture.

• This is not practical and culture is somewhat less sensitive than NAATs, so interpreting results will still be difficult.

• There is still a need for judgment in ordering and interpreting tests

ConclusionsConfirmatory testing is not indicated for all specimens that test

positive for N. gonorrhoeae using NAATs in low prevalence populations

• Confirmatory testing is not practical• Perceived need based on low threshold for absolute risk of false positive• Assumes test performance cannot be defined

• Need for confirmation depends on the test• Aptima performs well and that confirmatory testing is not needed.• PCR does not perform adequately and confirmatory testing is

needed.• Data are inadequate to assess the need for confirmatory

testing with SDA & Probetec – some data low positive SDA may

be more likely to be false positive