Objectives
Our main objective was to measure sensitivity, specificity, positive and negative predictive values, likelihood ratios of NT-proBNP blood concentration to suspect cardiac failure at post mortem examination. Our secondary objective was to measure correlations between NT-proBNP blood concentrations and those measured in other available fluids (serum, pericardial fluid, vitreous humor). Moreover, we aimed to quantify the eventual influence of age, sex, BMI, other diseases known to modify the NT-proBNP levels and traumatic death on the sensitivity and specificity of NT-proBNP blood levels in detecting heart failure.
Studied population
One hundred sixty-eight out of 210 consecutive medicolegal autopsies performed at the University Centre of Legal Medicine of Lausanne in 2006 were included in this study. To be included, standard investigation procedures (autopsy, histology, toxicology) had to be made and subjects had to be older than 15 years. The post mortem interval ranged from <12 to 24 h. Bodies with a post mortem interval >24 h were excluded from the study because putrefaction, at any stage, would have rendered impossible or at least very difficult to identify signs of heart failure at autopsy and thus to compare them with the NT-proBNP values measured in various body fluids most of which, moreover, would have been unavailable in case of putrefaction. All autopsies were demanded by legal authorities. The sample reflected the variability of cases seen in forensic post mortem evaluations, as it is illustrated in Table 1.
Table 1 Characteristics of subjects (n = 168)
Reference test
No gold standard is available in forensic setting. The reference test is the usual procedure used when all available investigations can be performed in usual forensic practice. The diagnosis of heart failure was based on autopsy findings including cardiac cavities dilatation with or without myocardial hypertrophy, visceral congestion of internal organs, lung and/or peripheral oedema. Moreover, the presence of referred or detected pre-existing pathologies known to be causally associated with this syndrome (as coronary artery disease, hypertension, obesity and valvular heart disease) was considered in corroborating the diagnosis of heart failure.
For each case, a complete autopsy was performed, followed by routine histological examination. Details for each case were extracted from the autopsy report and completed by complementary information (sequence of events, circumstances of body discovery, known medical story).
All autopsies were performed by seven physicians, following international recommendations [13]. Three of these physicians were senior residents with at least 10 years of experience in forensic medicine, and 4 of them were residents, always supervised by a senior. All these forensic pathologists were blinded to NT-proBNP lab results.
Two independent forensic pathologists assessed their appreciation of cardiac failure for each case using a four-item Likert scale (most unlikely, unlikely, very likely, certainly). For cases where experts disagreed, a third forensic pathologist gave his opinion and cases were discussed until a consensus was reached. In their evaluations, these examiners were blinded to NT-proBNP values. Subjects were considered as having suffered from cardiac failure if assessors agreed they were very likely or certainly affected by this condition.
Index test (NT-proBNP)
Index test was performed after the reference test (prospective). Femoral blood, serum, vitreous humor and pericardial fluid were collected during the autopsy. Serum was obtained by centrifugation of blood samples, immediately after collection. Samples were sent by the forensic examiner to the laboratory and the specimens were stored at −20°C until the time of analysis. Laboratory staff, who reported results, was completely blinded to information regarding cases, except for gender and age. NT-proBNP blood concentrations were measured using a chemiluminescent immunoassay kit (Elecsys 2010 analyser, Roche Diagnostic, Basel, Switzerland; Dimension Xpand plus, Dade Behring, Deerfield, IL, USA). The procedure’s test performances were the following: CV inter-assay 9% (490 ± 44 pmol/L), CV intra-assay 7.3% (431 ± 32 pmol/L) and 9.7% (1,400 ± 135 pmol/L). No cross-reactivity for human BNP, ANP or N-terminal proANP. LOD 9.7 pmol/l [7]. As serum is not always available in forensic setting (haemolysis), we chose blood as a reference standard, and we defined the clinical serum value of >112 pg/mL as cut-off also for NT-proBNP blood concentration. To test whether NT-proBNP blood values are correlated to those measured in serum, pericardial fluid and vitreous humor, the same laboratory method was applied to these fluids.
Other measures
Myocardial specimens for the histological examination were derived from at least five different topographic locations, namely left anterior, left lateral, left posterior, septal and right lateral. They were stained with hematoxylin–eosin. For each case, alcohol concentration was assessed in peripheral blood. Toxicological analyses were performed, when requested, by GC/MS. The presence of common abuse substances was assessed, including cocaine and its metabolites, and the results were compared to published data [14].
Statistical analyses
Receiver operating curve and area under the curve (AUC) were calculated for different NT-proBNP values. In the clinical setting, the reference ranges for NT-proBNP vary depending on the assay method used and the nature of the control population [15, 16]. A suggested “normal” range for NT-proBNP is 68–112 pg/mL [5, 16]. Thus, we chose a cut-off value of >112 pg/mL to define NT-proBNP as positive or negative. Sensitivity, specificity, positive and negative predictive values, positive and negative likelihood ratios were then given with 95% confidence intervals (95% CI). Sensitivity and specificity were calculated for sub-populations (age and sex, BMI, presence of renal, pulmonary and central nervous system pathology, signs of trauma). Linear regression analysis was used to define association between NT-proBNP concentrations in blood and in other biological fluids. Transformations were used to test other associations than linear correlations, when required. R
2 from the regression was calculated to estimate the strength of the association between blood and other body fluids. Analyses were conducted using STATA 10.1 (College Station, TX 77845 USA).