Background Specimens with incorrect patient information are both a critical safety error and difficult to identify. Estimates of sample mislabelling rely on subjective identification of mislabelling, with the possibility that not all mislabelled samples are being caught.

Methods We determined the blood type of two or more complete blood count specimens with the same patient label and assessed for discrepancies. We additionally determined the rate of identified sample mislabelling for the study period.

Results We found a rate of 3.17 per 1000 discrepancies over the study period. These discrepancies most likely represent occult, or unidentified, mislabelled samples. In contrast, the rate of identified sample mislabelling was 1.15 per 1000.

Conclusions This study suggests that specimens identified as, or known to be, mislabelled represent only a fraction of those mislabelled. These findings are currently being confirmed in our laboratory and are likely generalisable to other institutions.