患者男,76歲,因“頭暈、乏力3個月”來診。查體:中度貧血貌。血常規(guī):Hb76g/L,MCV73fl,MCH22pg,MCHC301g/L,WBC5.6×109/L,PLT437×109/L,網(wǎng)織紅細(xì)胞0.01。應(yīng)首先進(jìn)行的檢查是()