Sentence:  The Mecholyl and noradrenalin tests applied with certain precautions <WSD>are</WSD> reliable indicators of this central autonomic balance , but for the sake of correlating autonomic and clinical states , and of studying the effect of certain therapeutic procedures on central autonomic reactions , additional tests seem to be desirable . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)