If you PH pen has been calibrated properly.. it will be accurate.. Those drops give a "close estimate" and often the quality of the reading degrades with shelf life. If you were using the drops before and have nute lock out.. there is a good chance the drops are wrong.. if you just bought the pen.. try it for a while and see if the condition of your plants improve.. i can pretty much guarantee the drops are wrong.. and the pen will be right... Cheers!