Comparison of the tubercolin skin test and the ellispot blood test for the diagnosis of latent tuberculosis infection in pre-transplant dialysis patients

End stage renal disease (ESRD) is a known risk factor for progression to active tuberculosis (TB) from latent tuberculosis infection (LTBI). Kidney transplant and immunosuppressive therapy may increase the risk of TB recurrence. Patients with LTBI undergoing dialysis would therefore benefit from preventive treatment with isoniazid, which often has adverse side effects in this particular group of patients. Therefore it is important to accurately identify ESRD patients with LTBI, mainly if awaiting renal transplantation. The standard tool for diagnosing LTBI is the century-old tuberculin skin test (TST); however, patients with ESRD, as many other immunosuppressedpatients, often have falsely negative TST results. The Enzyme-Linked ImmunoSpot (ELISPOT) test is a new test which enumerates M. tuberculosis-specific T-cells in peripheral blood samples and has been already shown to be more specific and more sensitive than the TST for diagnosis of LTBI. We tested 84 ESRD patients on dialysis treatment with TST and ELISPOT: 26 were on peritoneal dialysis and 57 on hemodialysis; only 1 patient was on conservative treatment. Mean age was 48±14 years (range 23-75); 51 male and 33 female. Simultaneous RD1 Elispot and TST (5 UI PPD) were performed. According to current guidelines, the cut-off value for a positive TST using 5 UI of PPD is 10 mm; based upon previously published studies, the pre- defined positive cut-off for the ELISPOT is 20 spot forming cells per million peripheral blood mononuclear cells. In 64 patients (76%) TST and ELISPOT gave concordant results and in more than 90% were both negative. Only 2 patients tested TST-positive and ELISPOT-negative (one was an immigrant from a high-prevalence Country) while 18 (21%) were TST-negative, but ELISPOT positive. These preliminary results indicate that a significant proportion of ESRD patients on dialysis treatment may have hitherto unrecognised LTBI and an associated increased risk of progression to active disease, mainly after renal transplantation.