Fecal leukocyte stain has diagnostic value for outpatients but not inpatients

The methylene blue stain for fecal leukocytes (FL) is widely used as an adj unct to slower but more accurate tests of diarrheal etiology, such as stool culture (SCx) or toxin assays for Clostridium difficile. Prior studies inv estigating the utility of FL for predicting SCx and C. difficile toxin assa y (CDTA) results did not evaluate the importance of inpatient versus outpat ient status. We conducted a study of patients who submitted a stool specime n to the Stanford Hospital Microbiology Laboratory between May 1998 and Apr il 1999, The results for stool specimens that were tested by FL and by a co nfirmatory test (either SCx or CDTA) were used to determine whether the FL method helped to predict the results of these tests. Of 797 stools that wer e tested by FL method and at least one confirmatory test, 502 stools were t ested by CDTA, and 473 stools were cultured. The FL test was 14% sensitive and 90% specific for C, difficile with a diagnostic threshold of one white blood cell/high-power held (WBC/HPF). The overall likelihood ratio (LR) for a positive CDTA was 1.4 with a 95% confidence interval (CI) of 0.5 to 3.7 (P = 0.5) and was similar among inpatients and outpatients. In contrast, th e presence of greater than or equal to1 WBC/HPF was 52% sensitive and 88% s pecific for the 27 positive SCx results and helped to predict a positive SC x result (LR, 4.2; 95% CI, 2.7 to 6.5; P < 0.001). The sensitivity of <grea ter than or equal to>1 WBC/HPF was 57%, and its predictive value for SCx wa s higher among outpatients (outpatient LR, 5.0; 95% CI, 2.9 to 8.6; P < 0.0 01; inpatient LR, 1.9; 95% CI, 0.3 to 10.8; P = 0.5). Among inpatients, onl y 4 (1.5%) of the 273 SCx results were positive, and the presence of <great er than or equal to>1 WBC/HPF was insensitive (25%) and did not predict a p ositive SCx (LR, 1.9; 95% CI, 0.3 to 10.8; P = 0.5). When the data were rea nalyzed using a diagnostic threshold of five WBC/HPF for FL, the predictive power of the FL method was similar. Thus, FL was of no value in predicting CDTA positivity, nor was it helpful in predicting SCx results for inpatien ts. Neither SCx nor the FL method should routinely be performed on samples from inpatients. Among outpatients, presence of FLs should suggest a bacter ial diarrhea in clinically compatible cases.