Limited Reliability of Lipid-laden Macrophage Index Restricts Its Use as a Test for Pulmonary Aspiration: Comparison with a Simple Semiquantitative Assay

Abstract

The lipid-laden macrophage index (LLMI) is a semi-quantitative evaluation of alveolar macrophage lipid content used in diagnosis of pulmonary aspiration. To date, there are no published reports regarding the reliability of LLMI. We sought to evaluate the interobserver and intraobserver variability and validity of LLMI and to compare it to a simpler macrophage lipid content index (LCI). To evaluate reliability we compared both the LLMI and LCI of 26 bronchoalveolar lavage (BAL) specimens from 14 aspirators and 12 non-aspirators on 10 separate occasions by two observers. The ranges of means and standard deviations (SD) of LLMI for observer 1 (Obs 1) were 19–160 (5–31) for aspirators, and 0–48 (0–15) for non-aspirators; and those of observer 2 (Obs 2) were 77–249 (13–33) for aspirators and 47–170 (8–37) for non-aspirators. The ranges of means and SD of LCI for Obs 1 were 2–8 (0–2) for aspirators and 0–4 (0–1) for non-aspirators, compared with 2–9 (0–2) for aspirators and 1–6 (0–2) for non-aspirators for Obs 2. No statistical significance was found between LLMI and LCI by comparing coefficients of variation (CV) in either groups or observers. Poor agreement between the two observers was found using a Bland Altman analysis, with the difference of the two observations mostly exceeding zero and becoming larger as the average of the two observations became bigger. The combined sensitivity, specificity, and positive and negative predictive value (PPV and NPV) for both observers of the LLMI were 57%, 75%, 84%, and 69% and those of LCI were 58%, 92%, 93%, and 69%. We conclude that there is poor reliability for both methods. The LCI is simpler and appears to be at least as good as the LLMI.