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paired blood and urine samples from 39 patients who were consecutively referred to the outpatient clinic of the regional reference center for multiple sclerosis of the careggi university hospital , florence , italy , were prospectively collected .
the patients were recruited according to the following inclusion criteria : definite diagnosis of ms ; relapsing - remitting course ; fulfillment of the european medicines agency indications for natalizumab administration in ms ; and written informed consent to participate in a program for pml risk stratification established in collaboration with the marketing authorization holder ( mah ) of natalizumab and based on a centralized anti - jcpyv ab assay in sera and on a locally organized jcpyv dna analysis in blood and urine .
the results included in the present study were generated by analyzing the diagnostic data reported in the clinical records .
the study was approved by the local ethic committee ( comitato etico azienda ospedaliero - universitaria careggi , largo brambilla 3 , firenze ) according to the italian law requirements ( n. oss 14.073 ) .
the authors obtained preventive unrestricted permission for analyzing the data provided by the mah of the anti - jcpyv ab assay .
the jcpyv dna was extracted from peripheral blood mononuclear cells ( pbmc ) obtained from enriched buffy coats after density gradient separation of 8 ml of whole blood , 200 l of plasma , and 200 l of urine , and was quantified by jcpyv - specific real - time pcr targeting the large t gene performed in triplicate using 100 ng of dna ( forward primer 5-gcagcttagtgattttcttagg-3 , reverse primer 5-gaacacaggtgtttccacctgg-3 , and taqman mgb probe fam-5-ggcactgaatattcattcatgg3 ) .
each assay was carried out with negative controls ( no template ) and dna 10-fold dilutions ( 1010 copies ) of a plasmid carrying the amplicon cloned ( standard curve plot was 1010 copies vs 36 - 21 cycle threshold ) .
the lower limit of quantification of the jcpyv dna was 100 copies per ml of plasma and urine or per g of pbmc dna .
anti - jcpyv abs were detected in serum by a 2-step virus - like particle - based elisa assay ( stratify , focus diagnostics , provided by biogen idec ) .
patients who tested positive for jcpyv dna in blood or urine but who had undetectable serum antibodies had their serum samples retested for anti - jcpyv abs .
paired blood and urine samples from 39 patients who were consecutively referred to the outpatient clinic of the regional reference center for multiple sclerosis of the careggi university hospital , florence , italy , were prospectively collected .
the patients were recruited according to the following inclusion criteria : definite diagnosis of ms ; relapsing - remitting course ; fulfillment of the european medicines agency indications for natalizumab administration in ms ; and written informed consent to participate in a program for pml risk stratification established in collaboration with the marketing authorization holder ( mah ) of natalizumab and based on a centralized anti - jcpyv ab assay in sera and on a locally organized jcpyv dna analysis in blood and urine .
the results included in the present study were generated by analyzing the diagnostic data reported in the clinical records .
the study was approved by the local ethic committee ( comitato etico azienda ospedaliero - universitaria careggi , largo brambilla 3 , firenze ) according to the italian law requirements ( n. oss 14.073 ) .
the authors obtained preventive unrestricted permission for analyzing the data provided by the mah of the anti - jcpyv ab assay .
the jcpyv dna was extracted from peripheral blood mononuclear cells ( pbmc ) obtained from enriched buffy coats after density gradient separation of 8 ml of whole blood , 200 l of plasma , and 200 l of urine , and was quantified by jcpyv - specific real - time pcr targeting the large t gene performed in triplicate using 100 ng of dna ( forward primer 5-gcagcttagtgattttcttagg-3 , reverse primer 5-gaacacaggtgtttccacctgg-3 , and taqman mgb probe fam-5-ggcactgaatattcattcatgg3 ) .
each assay was carried out with negative controls ( no template ) and dna 10-fold dilutions ( 1010 copies ) of a plasmid carrying the amplicon cloned ( standard curve plot was 1010 copies vs 36 - 21 cycle threshold ) .
the lower limit of quantification of the jcpyv dna was 100 copies per ml of plasma and urine or per g of pbmc dna .
anti - jcpyv abs were detected in serum by a 2-step virus - like particle - based elisa assay ( stratify , focus diagnostics , provided by biogen idec ) .
patients who tested positive for jcpyv dna in blood or urine but who had undetectable serum antibodies had their serum samples retested for anti - jcpyv abs .
demographic and clinical characteristics and jcpyv infection status of the 39 patients with rrms are reported in table 1 .
the blood and urine samples were collected biannually at baseline and during the therapy , from july 2008 to september 2012 .
we included in the study the first positive isolation of jcpyv dna and the anti - jcpyv ab assay evaluated in the serum sample of the same patient collected concurrently or after the jcpyv dna isolation .
eleven out of 39 samples included were collected at baseline , and the other 28 were collected during natalizumab administration .
no obvious associations between patient demographic and clinical characteristics ( age , sex , disease duration , previous treatments ) and jcpyv dna status were observed .
overall , jcpyv dna was detected in at least one body fluid in 23 of 39 patients ( 59% ; table 2 and table e-1 at neurology.org/nn ) : 17 patients ( 44% ) had jcpyv dna in pbmc , 4 patients ( 10% ) in plasma , and 13 patients ( 33% ) in urine ( table 2 ) .
of note , the assay could detect jcpyv dna in 1 of 20 ( 5% ) blood ( pbmc or plasma ) samples and in 11 of 20 ( 55% ) urine samples of healthy donors .
the range of viral load was similar in the different compartments ( table 2 ) .
baseline demographic and clinical characteristics of the patients according to jcpyv status jcpyv dna and anti - jcpyv ab status in the patients with rrms included in the study anti - jcpyv abs were present in 25 of 39 patients included ( 64% ) and in 16 of the 23 jcpyv - infected patients ( 70% ; table 2 and table e-1 ) , indicating an assay sensitivity of 0.7 . in addition , jcpyv dna was present in 7 of the 14 anti - jcpyv antibody  negative patients ( 50% ; table 2 ) , indicating that the negative predictive value of the assay was 0.5 . however , in this context , the viral load observed in seronegative patients did not differ from that observed in the seropositive patients . of note , only 1 of the 7 patients who was anti - jcpyv ab  negative despite the presence of jcpyv dna in pbmc showed seroconversion in a sample taken within 6 months .
in the present study , the frequency of jcpyv dna ( 59% ) in patients with ms treated with natalizumab was higher than that previously reported when jcpyv infection was evaluated only in urine ( 24% ) and was consistent with that reported when the infection was evaluated in pbmc ( 60% ) .
the high frequency of jcpyv in pbmc , a cell compartment well - known for its ability to harbor jcpyv - dna , was not unexpected , particularly in patients treated with natalizumab , considering the effects of this drug on lymphoid precursor cells .
the frequency of anti - jcpyv abs observed in serum ( 64% ) was consistent with previous observations in similar populations .
however , in our study , anti - jcpyv abs were present in only 70% of the 23 patients with jcpyv infection .
this result puts the sensitivity of the stratify test at 70% , whereas in previous studies that used the same assay to detect anti - jcpyv abs sensitivity was reported to be 97% .
this difference should probably be attributed to the method of identifying the presence of the jcpyv dna , as the serologic data reported here and in previous studies have been obtained with the same methods ( the stratify test ) .
the previous data have been obtained by testing the virus dna only in urine , whereas more recent data and the present study indicate that when peripheral blood is also tested the frequency of jcpyv dna is higher than previously described , particularly during natalizumab treatment .
in addition , the present study data , which put the negative predictive value of the ab assay at 50% , are consistent with other recent observations reporting that when peripheral blood was included in the analysis the false - negative rate of jcpyv serology was 37% and 35% .
the present results indicate that jcpyv dna evaluation in blood and urine could detect jc virus infection earlier than anti - jcpyv ab serology .
thus , anti - jcpyv ab serology may underestimate jcpyv infection frequency , so it may be useful but not always sufficient to predict pml risk .
this possibility points out the need to reevaluate the pml risk thus far reported in anti - jcpyv seronegative patients through additional virologic investigations .
however , the limited number of patients included does not allow us to conclusively quantify the real frequency of anti - jcpyv ab 
this should be reevaluated in a larger set of patients identified by assessing jcpyv dna in paired blood and urine samples .
v. clausi contributed to data collection , data analysis , and interpretation of the data .
s. giannecchini contributed to data analysis , interpretation of the data , drafting of the manuscript , and revision of the manuscript .
e. magnani contributed to patient recruitment and follow - up and interpretation of the data .
a. repice contributed to patient recruitment and follow - up and collection , analysis , and interpretation of the data .
c. mechi contributed to patient recruitment and follow - up and collection , analysis , and interpretation of the data .
l. massacesi contributed to the design , interpretation of the data , and revision and supervision of the manuscript .
the work was supported by grants from the fondazione  istituto di ricerca virologica oretta bartolomei corsi  florence , italy .
v. clausi , s. giannecchini , e. magnani , a. repice , c. mechi , f. martelli , and a. azzi report no disclosures .
l. massacesi received reimbursements for meeting participation or educational grants from biogen idec , merck - serono , sanofi - aventis , and novartis .
l. massacesi is a member of the scientific advisory group ( sag ) neurology of the european medicines agency ( ema ) and of the italian medicines agency ( agenzia italiana del farmaco , aifa ) , but the opinions included in this article may not represent the positions of these agencies . go to neurology.org/nn for full disclosures .
objective : to evaluate the frequency of jc polyomavirus ( jcpyv ) infection and anti - jcpyv antibodies in patients with multiple sclerosis under natalizumab therapy.methods:presence of anti - jcpyv antibodies and jcpyv dna was analyzed in 39 patients with relapsing - remitting multiple sclerosis undergoing natalizumab therapy . 
 anti - jcpyv antibodies were evaluated in serum by a 2-step virus - like particle - based elisa assay ( stratify ) , and jcpyv dna was evaluated in peripheral blood mononuclear cells , plasma , and urine by quantitative pcr . 
 the anti - jcpyv antibodies were evaluated in serum samples collected at the same time or later than those collected for dna analysis.results:jcpyv dna was detected in 59% of patients , and anti - jcpyv antibodies were present in 67% . 
 jcpyv dna occurred more often in blood than in urine . 
 anti - jcpyv antibodies were observed in 70% of the jcpyv - infected patients , and jcpyv dna was detected in 50% of the patients without anti - jcpyv antibodies . 
 when jcpyv dna was investigated in blood and urine the frequency of infection was higher than previously described.conclusion:under these experimental conditions , with respect to the observed frequency of jcpyv infection , the sensitivity of the anti - jcpyv antibody assay was lower than expected .