Defining the True Sensitivity of Culture for the Diagnosis of Melioidosis Using Bayesian Latent Class Models

BACKGROUND: Culture remains the diagnostic gold standard for many bacterial infections, and the method against which other tests are often evaluated. Specificity of culture is 100% if the pathogenic organism is not found in healthy subjects, but the sensitivity of culture is more difficult to determine and may be low. Here, we apply Bayesian latent class models (LCMs) to data from patients with a single Gram-negative bacterial infection and define the true sensitivity of culture together with the impact of misclassification by culture on the reported accuracy of alternative diagnostic tests. METHODS/PRINCIPAL FINDINGS: Data from published studies describing the application of five diagnostic tests (culture and four serological tests) to a patient cohort with suspected melioidosis were re-analysed using several Bayesian LCMs. Sensitivities, specificities, and positive and negative predictive values (PPVs and NPVs) were calculated. Of 320 patients with suspected melioidosis, 119 (37%) had culture confirmed melioidosis. Using the final model (Bayesian LCM with conditional dependence between serological tests), the sensitivity of culture was estimated to be 60.2%. Prediction accuracy of the final model was assessed using a classification tool to grade patients according to the likelihood of melioidosis, which indicated that an estimated disease prevalence of 61.6% was credible. Estimates of sensitivities, specificities, PPVs and NPVs of four serological tests were significantly different from previously published values in which culture was used as the gold standard. CONCLUSIONS/SIGNIFICANCE: Culture has low sensitivity and low NPV for the diagnosis of melioidosis and is an imperfect gold standard against which to evaluate alternative tests. Models should be used to support the evaluation of diagnostic tests with an imperfect gold standard. It is likely that the poor sensitivity/specificity of culture is not specific for melioidosis, but rather a generic problem for many bacterial and fungal infections

Factors Predicting and Reducing Mortality in Patients with Invasive Staphylococcus aureus Disease in a Developing Country

BACKGROUND: Invasive Staphylococcus aureus infection is increasingly recognised as an important cause of serious sepsis across the developing world, with mortality rates higher than those in the developed world. The factors determining mortality in developing countries have not been identified. METHODS: A prospective, observational study of invasive S. aureus disease was conducted at a provincial hospital in northeast Thailand over a 1-year period. All-cause and S. aureus-attributable mortality rates were determined, and the relationship was assessed between death and patient characteristics, clinical presentations, antibiotic therapy and resistance, drainage of pus and carriage of genes encoding Panton-Valentine Leukocidin (PVL). PRINCIPAL FINDINGS: A total of 270 patients with invasive S. aureus infection were recruited. The range of clinical manifestations was broad and comparable to that described in developed countries. All-cause and S. aureus-attributable mortality rates were 26% and 20%, respectively. Early antibiotic therapy and drainage of pus were associated with a survival advantage (both p<0.001) on univariate analysis. Patients infected by a PVL gene-positive isolate (122/248 tested, 49%) had a strong survival advantage compared with patients infected by a PVL gene-negative isolate (all-cause mortality 11% versus 39% respectively, p<0.001). Multiple logistic regression analysis using all variables significant on univariate analysis revealed that age, underlying cardiac disease and respiratory infection were risk factors for all-cause and S. aureus-attributable mortality, while one or more abscesses as the presenting clinical feature and procedures for infectious source control were associated with survival. CONCLUSIONS: Drainage of pus and timely antibiotic therapy are key to the successful management of S. aureus infection in the developing world. Defining the presence of genes encoding PVL provides no practical bedside information and draws attention away from identifying verified clinical risk factors and those interventions that save lives

Survey of innate immune responses to Burkholderia pseudomallei in human blood identifies a central role for lipopolysaccharide.

B. pseudomallei is a gram-negative bacterium that causes the tropical infection melioidosis. In northeast Thailand, mortality from melioidosis approaches 40%. As exemplified by the lipopolysaccharide-Toll-like receptor 4 interaction, innate immune responses to invading bacteria are precipitated by activation of host pathogen recognition receptors by pathogen associated molecular patterns. Human melioidosis is characterized by up-regulation of pathogen recognition receptors and pro-inflammatory cytokine release. In contrast to many gram-negative pathogens, however, the lipopolysaccharide of B. pseudomallei is considered only weakly inflammatory. We conducted a study in 300 healthy Thai subjects to investigate the ex vivo human blood response to various bacterial pathogen associated molecular patterns, including lipopolysaccharide from several bacteria, and to two heat-killed B. pseudomallei isolates. We measured cytokine levels after stimulation of fresh whole blood with a panel of stimuli. We found that age, sex, and white blood cell count modulate the innate immune response to B. pseudomallei. We further observed that, in comparison to other stimuli, the innate immune response to B. pseudomallei is most highly correlated with the response to lipopolysaccharide. The magnitude of cytokine responses induced by B. pseudomallei lipopolysaccharide was significantly greater than those induced by lipopolysaccharide from Escherichia coli and comparable to many responses induced by lipopolysaccharide from Salmonella minnesota despite lower amounts of lipid A in the B. pseudomallei lipopolysaccharide preparation. In human monocytes stimulated with B. pseudomallei, addition of polymyxin B or a TLR4/MD-2 neutralizing antibody inhibited the majority of TNF-α production. Challenging existing views, our data indicate that the innate immune response to B. pseudomallei in human blood is largely driven by lipopolysaccharide, and that the response to B. pseudomallei lipopolysaccharide in blood is greater than the response to other lipopolysaccharide expressing isolates. Our findings suggest that B. pseudomallei lipopolysaccharide may play a central role in stimulating the host response in melioidosis

Association between patient characteristics and outcome for 270 patients with <i>S. aureus</i> infection.

<p>Data are number (%) unless otherwise stated.</p>*<p>¹p value for the comparison between all-cause deaths and survivors.</p>*<p>²Denominator for occupation is number of patients over the age of 16 years which is given in each square.</p>*<p>³Past medical history of any underlying chronic medical conditions reported by the patient/relative or recorded in the medical notes.</p>*<p>⁴Immunosuppression from HIV (5 untreated, 3 on anti-retroviral therapy), chemotherapy (n = 3), untreated leukaemia (n = 1), radiotherapy (n = 1) or immunosuppressive medication including prednisolone more than 30 mg/day for more than 1 week (n = 17).</p>*<p>⁵Renal disease included end stage renal failure on long-term dialysis (n = 3; 2 on haemodialysis, 1 on peritoneal dialysis) and chronic renal failure (not on dialysis) due to diabetes mellitus (n = 14), systemic lupus erythematosus (n = 1), multiple myeloma (n = 1), glomerulonephritis (n = 1) or an unknown aetiology (n = 5).</p>*<p>⁶Cardiac disease comprised congenital heart disease (n = 4), valvular heart disease including rheumatic heart disease (n = 8), ischaemic heart disease (n = 8), or arrhythmias including heart block requiring pacemaker (n = 4).</p>*<p>⁷Lung disease comprised previously treated tuberculosis (n = 9), previous empyema (n = 1), lung cancer (n = 2), long-term tracheostomy (n = 1), chronic obstructive pulmonary disease (n = 2) or asthma (n = 1).</p

Significant risk factors for mortality from <i>S. aureus</i> infection from multiple logistic regression analysis.

*<p>¹95% confidence intervals.</p>*<p>²p value from Likelihood ratio test.</p

The range of sites of infection in patients and outcome associated with each clinical presentation.

*<p>¹p value for the comparison between all-cause deaths and survivors.</p>*<p>²Site of deep abscesses were muscle (n = 20), retroperitoneal space (n = 7), parotid gland (n = 7), liver (n = 3), lung (n = 2), epidural space (n = 2), eye (n = 2), oropharynx (n = 2) and spleen (n = 1).</p>*<p>³Other skin and soft tissue infections includes: necrotising fasciitis (n = 9), bedsore(s) (n = 6), pustules and carbuncles (n = 5), infected wound from trauma (n = 3), infected wound from tophi (n = 2), gangrene (n = 2), cellulitis (without other skin or soft tissue lesion) (n = 2) and infection of exfoliated skin following a severe drug reaction (n = 2).</p>*<p>⁴Orthopaedic material includes: internal fixation metalwork (n = 8) and a hip replacement (n = 1).</p>*<p>⁵Intravenous devices were peripheral cannulas (n = 4), central catheters (n = 3) and an umbilical catheter (n = 1).</p>*<p>⁶Endocarditis from transthoracic echocardiographic evidence of vegetations (n = 7); 1 case clinically but died prior to echocardiogram.</p>*<p>⁷Other infections include: urinary tract infection (n = 3), tenosynovitis (n = 2), Lemierre's syndrome (n = 1) and corneal ulcer (n = 1).</p>*<p>⁸Post-operative infections include: mediastinitis (n = 4; 3 following mitral valve replacement and 1 after coronary artery bypass graft), meningitis from infected bone flap surgical wound (n = 1) and abdominal wound (n = 1).</p

Timely effective antibiotic therapy and procedures for infectious source control significantly improved outcome.

<p>Administration of an effective antibiotic on the same day as the positive culture was taken significantly reduced all-cause mortality (p<0.001), as did undergoing a procedure for infectious source control (p<0.001).</p

Higher all-cause mortality associated with methicillin-resistant <i>S. aureus</i> (MRSA) but not with Panton-Valentine Leukocidin (PVL).

<p>Patients infected by MRSA had a greater all-cause mortality compared with patients infected by methicillin-susceptible <i>S. aureus</i> (MSSA) (p<0.001). Conversely, patients infected by PVL gene-positive <i>S. aureus</i> had a lower all-cause mortality compared with patients infected by PVL gene-negative <i>S. aureus</i> (p<0.001), an association that remained after adjustment for MRSA (p = 0.001).</p

WBC-normalized plasma cytokine concentrations induced by stimulation of whole blood from 300 healthy subjects at 37°C for six hours with medium alone, <i>E. coli</i> O111:B4 LPS 10 ng/ml (as a positive control), heat-killed <i>B. pseudomallei</i> 1026b 2.5 × 10⁶ CFU/ml, or heat-killed <i>B. pseudomallei</i> K96243 2.5 × 10⁶ CFU/ml.

<p>Boxes show the median and interquartile range; whiskers show upper and lower adjacent values; outside values are not shown for clarity.</p