Toluidine blue cytometry test for sperm DNA conformation: comparison with the flow cytometric sperm chromatin structure and TUNEL assays

Background: Sperm DNA integrity (SDI) is an important factor in the prognosis of male fertility. Here we compare the toluidine blue (TB) image cytometry test, recently proposed by us for SDI assessment, with two other tests - the sperm chromatin structure assay (SCSA) and the terminal nick-end labelling (TUNEL) assay. Methods: Sperm samples from 35 men were evaluated for standard sperm parameters and subjected to the TB test and SCSA. Eighteen of the 35 samples were also subjected to the TUNEL assay. Results: The proportion of sperm cells with abnormal DNA integrity assayed by the TB test correlated strongly with the proportion of abnormal cells detected by the SCSA and TUNEL assay (ρ = -0.84 and ρ = 0.80, P < 0.001, respectively). Furthermore, the fractions of abnormal cells by the TB test corresponded closely to the sum of two SCSA parameters, the DNA fragmentation index (DFI) and the fraction of highly DNA-stainable cells (HDS) (medians 33.0 versus 32.0%, P = 0.6). Conclusions: Abnormal cells in a TB test correspond to the sum of DFI and HDS fractions in the SCSA. TB-positive cells may represent sperm with fragmented DNA and/or abnormal chromatin structure. Because the TB test is an easy and inexpensive method, its potential use as a routine test for sperm DNA integrity, complementary to standard semen parameters, should be investigated further.