from march 2003 though november 2004 , admitting physicians in district and provincial hospitals within 9 provinces of thailand in the north , northeast , central , and southern regions were invited to recruit patients of all ages suspected on clinical grounds to have leptospirosis . clinical features considered were those specifically referred to in the national guidelines ( e.g. , fever , headache , muscle pain , meningism , conjunctival suffusion , and jaundice ) together with hemoptysis , hepatomegaly , diarrhea , hypotension , and reduced urine output . from each patient , a 5-ml serum sample was taken to be cultured for leptospira , another 5-ml serum sample was taken for serologic testing , and a third sample was taken 2 weeks later for serologic testing . . microscopic agglutination test ( mat ) was performed at the world health organization ( who)/united nations food and agriculture organization ( fao)/world animal health organisation ( oie ) collaborating center for reference and research on leptospirosis , brisbane , queensland , australia ( 4 ) . a positive mat was defined as a single titer of > 1:400 or a 4-fold rise in titer between acute and convalescent phase samples . for leptospira culture , 100 l of whole blood , 500 l of plasma , and 500 l of serum were each injected into 3 ml of ellinghausen , mccullough , johnson , and harris ( emjh ) medium and supplemented with 3% rabbit serum and 0.1% agarose , then incubated aerobically at room temperature ( 25c30c ) for 6 months and examined every week for 2 months , every 2 weeks during months 3 and 4 , and once a month during months 5 and 6 . examination was done by placing 1 drop of culture onto a microscopic glass slide and viewing by dark - field microscopy at 200 magnification . positive cultures were referred to the who / fao / oie collaborating center for reference and research on leptospirosis for identification by using the cross - agglutination absorption test ( 4 ) . a total of 700 patients with a clinical diagnosis of leptospirosis were recruited during the study period . all patients had blood samples collected at the hospital for leptospire culture and serologic testing ; convalescent - phase serum samples were obtained during follow - up for 509 ( 73% ) patients . the median age of patients with suspected leptospirosis was 38 years ( range 295 years , interquartile range ( iqr ) 2849 years ) ; 504 ( 72% ) were men . the number of clinically diagnosed leptospirosis cases by month in the north , northeast , central , and southern regions is shown in figure 1 . most cases ( 597 , 85% ) were recorded in 4 provinces in the north or northeast ( table ) . cases were predominantly identified during the rainy season ( june  october ) in the north and northeast in 2003 , with a second peak in the northeast , but not the north , during the rainy season of 2004 . cases of clinical leptospirosis by month for each geographic region , thailand , march 2003november 2004 . ci , confidence interval . of the 700 patients who received a clinical diagnosis of leptospirosis , 143 ( 20% ) received a confirmed diagnosis of leptospirosis based on leptospira isolation , mat testing , or both ( table ) . the median age of patients with confirmed leptospirosis was 35 years ( range 1068 years , iqr 2745 years ) ; 121 ( 85% ) were men . the diagnosis was confirmed after isolation of leptospires from 15 ( 11% ) patients ; the geographic distribution is shown in the table . the serovars of cultured leptospira were : l. interrogans serovar ( sv . ) autumnalis ( 7 ) , l. interrogans sv . hebdomadis ( 1 ) , l. interrogans sv grippotyphosa ( 1 ) , and an unidentified serovar ( 1 ) . an additional 128 patients with culture - negative samples had been exposed to leptospira as determined by mat ; results for 96 ( 75% ) were based on a 4-fold rising titer and for 32 ( 25% ) , on a single raised titer of > 1:400 . the geographic distribution of the 143 laboratory - confirmed cases is summarized in the table . most of these patients ( 124 , 87% ) lived in the 4 provinces found in the north and the northeast ( table ) . the month of diagnosis for confirmed cases is shown in figure 2 ; most were during the rainy season . cases of laboratory confirmed leptospirosis and positive predictive accuracy of clinical diagnosis by month , thailand , march 2003november 2004 . the positive predictive accuracy of a clinical diagnosis is defined by the number of laboratory - confirmed cases divided by the number of clinically suspected cases . results for each of the 9 provinces are shown in the table . when only data from centers that reported at least 10 cases were used , positive predictive accuracy ranged from 3% to 29% . diagnosing leptospirosis at the point of care is notoriously difficult in the tropical setting , where several common infectious diseases are often hard to differentiate . positive predictive accuracy for leptospirosis was highest during the rainy season , an observation that is likely related to the higher disease incidence and pretest probability . variability in positive predictive accuracy was seen among the 3 provinces with the highest number of both suspected and true cases . the reason for this is unclear but may relate to perceived risk to the community , local policy , or other factors . the finding that both clinical and confirmed cases of leptospirosis were more common in the north and northeast is consistent with ddc reports . increased incidence in this region may have resulted from > 1 events , such as an increase in the rodent population , a natural reservoir for this pathogen , and a population in which around one third are positive for leptospira in northeast thailand ( 5 ) . alternatively , 1 clone or a small number of bacterial clones may have become adapted for persistence at greater numbers within the natural host or in the environment . it is also possible that 1 clone or a small number of clones have become adapted for enhanced invasion of the human host . the most prevalent serovar isolated was l. interrogans serovar autumnalis ( 7/15 [ 47% ] isolates ) , 6 of which were from cases in the north or northeast . the effect of the low level of accuracy of hospital - based clinical diagnosis of leptospirosis in rural thailand is not known . a common disease in this setting that is easily confused with leptospirosis is scrub typhus ; both diseases would be predicted to respond to doxycycline , an antimicrobial drug often prescribed for undifferentiated fever . further studies are required to define the implications of our findings and determine whether routine laboratory testing for leptospirosis should be implemented in thailand .