[Bacteremia and specific antibody response. IgG and IgM specificity in the evaluation of positivity in blood cultures].

Specific serum IgG and IgM directed against blood culture isolated bacteria have been determined in patients with positive blood culture by the indirect immunofluorescence and passive hemagglutination methods in order to distinguish the true positive blood cultures due to bacteremia from those due to contamination. 45 (16%) out of 280 blood cultures examined during the period 1/1/1980-30/1/1981 gave positive isolations: 25 cases (55.5%) were due to gram-negative bacteria and the remaining 20 cases (44.5) to gram-positive bacteria: among these there were 9 cases (20% of the total positive blood cultures) with isolation of micrococci. No positive blood culture for anaerobic bacteria were observed during that time interval. 96% of patients from whose blood cultures gram-negative bacteria had been isolated showed high titres of serum specific antibodies with both the indirect immunofluorescence and the passive hemagglutination methods. Only 40% of the cases whose blood cultures gave isolation of gram-positive bacteria showed high titres of serum specific IgG. The remaining 60% that did not show presence of serum specific antibodies included 9 cases of positive blood culture for micrococci (well known as contaminants) and 2 cases for streptococcus (in two of these there had been a mixed flora isolation). Specific IgM antibodies at significant level were also present in 76% of patients with positive blood culture for gram-negative bacteria and in 40% of patients with positive blood culture for gram-positive bacteria. All patients whose cultures gave isolation of micrococci showed absence of specific IgM. The observed good correspondence between isolation of contaminant bacteria on one hand and the absence of serum specific antibodies on the other, and vice versa between isolation of pathogenic bacteria (either obligate or opportunistic) an one hand and the presence of high level of specific antibodies on the other suggests that the search of specific immune response in patients with positive blood culture might constitute a good criterion (in addition to the classical criteria) for distinguishing true positive blood cultures from contaminated blood cultures.