tb genotype clusters were defined as groups of > 3 tb case - patients whose isolates had matching spoligotyping and 12-locus mycobacterial interspersed repetitive unit  variable number tandem repeat ( miru - vntr ) ( 9 ) genotyping results that were reported in the same county within washington . a log - likelihood ratio ( llr ) was calculated for each genotype cluster identified during each of the two 3-year periods ( figure ) . the larger the llr , the greater the possibility the cluster represented geographically concentrated tb cases , a proxy for recent tb transmission . the cutoff point for the llr was set to 5.0 , based on the value used by the national tb genotyping information management system ( 10 ) . formula used to calculate geospatial statistic ( a modified log - likelihood ratio [ llr ] ) on the basis of geographic distribution of mycobacterium tuberculosis genotype clusters , washington , usa . variables are classified as follows : a = number of tuberculosis ( tb ) cases with the genotype of interest in the selected county ; b = number of cases with the genotype of interest in the united states ; c = number of cases without the genotype of interest in the selected county ; d = number of cases without the genotype of interest in the united states ; n = total number of tb cases . qualitative analysis came from a 5-member expert panel of tb public health officials in washington . in 2008 , the panel participated in a discussion of all county - level tb clusters , ranking each as high or low priority for additional investigation . priority was determined on the basis of a review of patient characteristics , epidemiologic links from field investigations , and maps of genotype distributions . the panel also had information from enhanced contact investigations from local public health investigation teams that included the ability to order is6110 restriction fragment - length polymorphism ( is6110 rflp ) and 24-locus miru - vntr testing for clusters of concern , but results from these tests were not universally available . the ranking exercise with the same 5-member panel was repeated after period 1 for clusters from period 1 . llrs were compared with the expert opinion ranking to assess concordance . with expert opinion as the standard , negative and positive predictive values ( npv and ppv , respectively ) were calculated for period 1 using a cutoff point of llr > 5.0 . sensitivity and specificity of the > 5.0 llr cutoff point and exact binomial 95% cis were calculated for period 1 clusters . of 659 culture - positive cases , 642 ( 97.4% ) had genotyped isolates ; of these , 318 cases formed 21 clusters . five of these clusters had a high llr ; the expert panel ranked all 5 of these clusters high priority and identified them as clusters of concern . of the 16 clusters with llr < 5.0 , the expert panel ranked 12 ( 75.0% ) as low priority ( table ) . * llrs > 5.0 ( boldface ) indicate greater possibility that the cluster represents geographically concentrated tb cases , a proxy for recent tb transmission ) . variable number tandem repeat ; llr , log - likelihood ratio , a measure of geospatial concentration ; h , homelessness ; fb , foreign born ; sa , substance abuse .  period 1 , january 1 , 2005december 31 , 2007 ; the expert panel reexamined the 21 clusters identified during period 1 and focused on new activity within those areas during period 2 . two clusters with a high llr during period 1 continued to have a high llr during period 2 ; the expert panel continued to rank these high priority . the 3 other clusters that had a high llr during period 1 had a low llr for period 2 ; one of those was still considered a high priority by the panel . of the remaining 16 clusters , which continued to have a llr < 5 , 14 ( 87.5% ) were still ranked low priority by the panel . two clusters in the same county , pcr00309 and pcr00803 , had low llrs but were considered high priority by the expert panel . for cluster pcr00309 , the panel cited high levels of homelessness among case - patients as reason to rank it high priority . for cluster pcr00803 , the panel cited a highly mobile population from a tb - endemic country that regularly traveled into and out of the united states as reason to rank it high priority . however , the travel history among case - patients in this cluster made it difficult for investigators to determine whether transmission was occurring within washington or abroad . the npv and ppv for a llr cutoff point of 5.0 were 75% and 100% , respectively . lowering the cutoff point to a llr > 2.0 increased the npv to 92.3% , but the ppv remained at 100% . for period 1 clusters only , a cutoff point of llr > 5.0 generated a sensitivity of 55.6% ( 95% ci 21.2%86.3% ) and specificity of 100% ( 95% ci 73.5%100.0% ) for identifying clusters for further investigation . decreasing the cutoff point to > 2.0 increased the sensitivity to 88.9% ( 95% ci 51.8%99.7% ) but did not change the specificity ( 100% ; 95% ci 73.5%100.0% ) . the geospatial statistic in this study was highly correlated with experts perceived need for public health action . this finding indicates that automated alerts generated on the basis of geospatial concentration of tb cases might help the state tb program identify clusters that would benefit from additional investigation . automated alerts can be generated by using routinely collected surveillance data and are currently part of the national tb genotyping information management system ( 10 ) . patient and contact characteristics , transmission venues , and temporality all contribute toward prioritization determination . for example , during period 1 , a total of 6 ( 66.6% ) clusters ranked high priority by the expert panel were characterized by homelessness or substance abuse among case - patients , and 8 ( 88.9% ) were characterized by us - born case - patients ( table ) . conversely , 11 ( 91.7% ) clusters ranked low priority were characterized by case - patients who were foreign - born , a known risk factor for latent tb infection ( 7 ) . none of the period 1 clusters with llr > 5 and only 1 of 9 clusters ranked as high priority by the expert panel were characterized by foreign - born case - patients . these results indicate the need for further study to identify the limitations of the llr score in detecting localized and recent tb transmission among foreign - born case - patients . the availability of is6110 rflp or 24-locus miru - vntr testing results to the expert panel is the current standard for fieldwork and could have introduced an information bias for the panel in this study . although this effect is unknown , lack of universal is6110 rflp and 24-locus miru - vntr test results is a limitation of this study . we found that geospatial statistics based on tb genotyping and surveillance data could help identify and prioritize likely recent disease transmission events in washington . in addition , llr values should be incorporated into ongoing evaluation by the expert panel ; in fact , llr is now included in routine genotype and cluster reviews . geospatial statistics are an attractive approach to prioritization , but additional field - based research is needed to assess whether factors such as epidemiologic characteristics could be used to further develop a prioritization algorithm . integrating these factors and determining ideal cutoff points in different settings will increase predictive value .
groups of tuberculosis cases with indistinguishable mycobacterium tuberculosis genotypes ( clusters ) might represent recent transmission . 
 we compared geospatial concentration of genotype clusters with independent priority rankings determined by local public health officials ; findings were highly correlated . 
 routine use of geospatial statistics could help health departments identify recent disease transmission .