We use 0.90% w/v blood bank saline 20 L from Thermo (formly known as Fischer) for all our blood banking needs. We are a small transfusion service and we use the 20 L in about 3 weeks. As part of our weekly maintenance for cellwashers we take the pH of the saline to ensure it is still acceptable, we have been performing this maintenance for 6 months. The box of saline states that pH should remain between 6.0 and 7.5 and we have never had a problem with pH. The lowest I've seen it go is 6.1 (just before the box was done) otherwise it sticks around 7.0. Based on our historical data, if you keep your saline around for a month, you should be ok, you'd need to validate, however, it is feasible for the saline to stay stable for your bench work and your fetal screen. If you're worried about the pH for the fetal screens, perform a pH on your saline before you start your work.
