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A total of 548 strains of the eleven most common urinary tract pathogens were investigated for possible errors in norfloxacin susceptibility tests comparing MIC determinations with disk diffusion assays. Most strains were found to be sensitive with MIC‐90 values below 1.0 for the Enterobacteriaceae while the classical nalidixic acid resistant species, the gram‐positive bacteria and Pseudomonas aeruginosa, were less susceptible to norfloxacin with MIC‐90 above 1.0 mg/l. MIC‐values close to interpretive MIC‐limits were recorded for S. faecalis and S. agalactiae using the recommendations of the national Committee for Clinical Laboratory Standards (NCCLS) (susceptible, S ≤ 4.0) and for P. aeruginosa and S. aureus using the Swedish Reference Group for Antibiotics (SRGA) standards (S ≤ 1.0). Susceptibility interpretations for these species showed a lack of accuracy consistent with methodological problems of reproducibility, an error called type I. The changes in the MIC‐limits required for these strains to correct the error would be S ≤ 4 for P. aeruginosa and S. aureus, S ≤ 8 for S. agalactiae and S ≤ 0.5 for S. faecalis. A type II error, occurring when a bacterial species shows a regression line different from the regular line, was also identified for S. saprophyticus. The use of breakpoints derived from single strains regression analysis corrected this error and also reduced the frequency of similar misinterpretations in other species. The term “species‐specific MIC‐limits” should be introduced along with the established concept of “species‐specific interpretive zone breakpoints” to allow for the correction of type I interpretive errors. Type II errors can be corrected by using species‐specific interpretive breakpoints, either issued by reference laboratories or derived by calculations from single‐strain regression analysis in the individual laboratory.

In order to identify the cause of septicemia and the resistance patterns of bacteria in Swedish patients with hematological disorders, all positive blood cultures collected at a hematological ward during 1980-1986 were evaluated retrospectively. 198 episodes of septicemia in 129 patients were recorded. 54% were males and 46% women with a median age of 67 years (range 16-88). Patients with acute leukemia (46%), lymphoma (19%) and myeloma (19%) dominated. The absolute neutrophil count (ANC) was less than 0.5 x 10(9)/l in 76% of the bacteremic episodes. A total of 253 consecutive isolates were found with 53% Gram-negatives and 47% Gram-positives. The dominating pathogens were Escherichia coli (27%), klebsiella/enterobacter (15%), pseudomonas (7%), coagulase negative staphylococci (13%), alpha-streptococci (13%), Staphylococcus aureus (10%) and anaerobes (6%). Coagulase negative staphylococci showed a significant increase in isolation rate during the study period. The majority of E. coli were resistant to ampicillin. The susceptibility of klebsiella/enterobacter to ceftazidime and cefuroxime was reduced, while no imipenem resistant strains occurred. Among coagulase negative staphylococci 61% were resistant to isoxazolylpenicillin, none to vancomycin. No dramatic changes in the etiology of septicemia or the susceptibility pattern during the study period were noticed. Coagulase negative staphylococci, S. epidermidis in particular, constitute an increasing problem among granulocytopenic patients.