Are the occasional aneuploid cells in peripheral blood cultures significant?

Cytogenetic results of 1,500 consecutive clinical cases from a young population were analyzed for rare cells with hypermodality (≥ 47 chromosomes) or hypomodality (≤ 45 chromosomes). Such instances of non‐modal chromosome gains or losses were random relative to referral diagnosis or modal karyotype. However, chromosome loss was correlated with size, smaller chromosomes being lost more frequently (correlation coefficient = 0.794). Sex chromosome gain or loss in vitro was of particular interest since mosaicism in vivo is frequently found in patients presenting with manifestations of Turner or Klinefelter syndrome. Cases with a referral diagnosis of sex chromosome abnormality showed no increased gain or loss of an X or Y chromosome when compared to other types of clinical cases. Our analyses suggest that when one non‐modal cell is found with a gain or loss of a chromosome relevant to the referral diagnosis, then the results on a count of 40 cells should differentiate in vitro artifact from probable in vivo mosaicism with 95% degree of confidence.