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 (greater than or equal to 47 chromosomes) or hypomodality (less than or equal to 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.