Reevaluation of Lyme serologic quantitative test indexes: confirmation that high first-tier test index values predict a positive second-tier result in a modified 2-tier Lyme testing algorithm.

OBJECTIVES: In a previous study, we reported that using a first-tier quantitative Lyme serologic index may obviate the need for second-tier confirmatory testing in samples above a defined cutoff value. Here we have expanded on the sample size to confirm and better refine the potential utility of the serologic index to aid in timely clinical decision-making.

METHODS: We reviewed 1 year of Lyme serologic test results sent to our laboratory for testing using a modified 2-tier testing algorithm (MTTT) to determine the probability of second-tier confirmation based on the initial Lyme index value.

RESULTS: Using the original Lyme index cutoff at which 100% of samples confirmed on second-tier testing (2.68), in the expanded dataset, 98.7% of samples confirmed at this cutoff. Using all of the expanded sample set data, we defined 6 Lyme index brackets from highly positive to equivocal to estimate the probability of the first-tier test confirming on second-tier testing. We also constructed a formula relating the index to the probability of confirmation across the continuum of index values to predict second-tier confirmation in an MTTT algorithm.

CONCLUSIONS: The use of a Lyme test index value may eliminate the need for confirmatory testing on many positive first-tier samples in patients suspected of having Lyme disease. First-tier Lyme test results may be improved by reporting the index value along with the probability of confirmation. This may facilitate more timely diagnosis and clinical decision-making and potentially improve the time to antimicrobial therapy.