Researchers used blood samples, saliva and semen Only one type. Which is why the use of only one type of secretions, for example, to reduce the variability (Possible confounders) in the trials.
The researchers used a sample of blood, saliva and semen samples of one kind only. The reason for using fluids only one kind of example. To reduce variability (Possible confounders) in the experiment.
The use of samples, blood, saliva and semen of one sample only. The reason of using only one type of biological fluid samples to reduce the variance (Possible confounders) in the experiment.