Diagnostic performances of manual and automated reticulocyte parameters in anaemic cats

Abstract

Objectives The objective of this study was to evaluate the diagnostic performances of manual and instrumental measurement of reticulocyte percentage (Ret%), reticulocyte number (Ret#) and reticulocyte production index (RPI) to differentiate regenerative anaemia (RA) from non-regenerative anaemia (NRA) in cats. Methods Data from 106 blood samples from anaemic cats with manual counts (n = 74; 68 NRA, six RA) or instrumental counts of reticulocytes (n = 32; 25 NRA, seven RA) collected between 1995 and 2013 were retrospectively analysed. Sensitivity, specificity and positive likelihood ratio (LR+) were calculated using either cut-offs reported in the literature or cut-offs determined from receiver operating characteristic (ROC) curves. Results All the reticulocyte parameters were significantly higher in cats with RA than in cats with NRA. All the ROC curves were significantly different ( P <0.001) from the line of no discrimination, without significant differences between the three parameters. Using the cut-offs published in literature, the Ret% (cut-off: 0.5%) was sensitive (100%) but not specific (<75%), the RPI (cut-off: 1.0) was specific (>92%) but not sensitive (<15%), and the Ret# (cut-off: 50 × 10³/µl) had a sensitivity and specificity >80% and the highest LR+ (manual count: 14; instrumental count: 6). For all the parameters, sensitivity and specificity approached 100% using the cut-offs determined by the ROC curves. These cut-offs were higher than those reported in the literature for Ret% (manual: 1.70%; instrumental: 3.06%), lower for RPI (manual: 0.39; instrumental: 0.59) and variably different, depending on the method (manual: 41 × 10³/µl; instrumental: 57 × 10³/µl), for Ret#. Using these cut-offs, the RPI had the highest LR+ (manual: 22.7; instrumental: 12.5). Conclusions and relevance This study indicated that all the reticulocyte parameters may confirm regeneration when the pretest probability is high, while when this probability is moderate, RA should be identified using the RPI providing that cut-offs <1.0 are used.

Figure 1

Flow diagram illustrating the selection procedure employed in this study. Ret = reticulocyte

Figure 2

Distribution of reticulocyte percentage (Ret%), absolute number of reticulocytes (Ret#) and reticulocyte production index (RPI) in cats with regenerative anaemia (RA) and non-regenerative anaemia (NRA), after (a) manual or (b) automated counts. The boxes indicate the I–III interquartile range (IQR), the horizontal line the median value, and the whiskers extend to further observation within quartile 1 minus 1.5 × IQR or to further observation within quartile 3 plus 1.5 × IQR. The grey area represents the reference interval of the laboratory. For each parameter and method, results of the RA group were significantly higher (P <0.001) than results of the NRA group

Figure 3

Comparison of receiver operating characteristic curves of the reticulocyte percentage (grey circle), absolute number of reticulocytes (black circle) and reticulocyte production index (open circle) obtained after (a) manual count or (b) instrumental count. The grey line indicates the line of no discrimination