Is it unusual to have isolated LDH elevation and normal HCG and AFP in non-seminomatous germ cell tumor (embryonal carcinoma)?

It's hard to generalize: the details of the case matter. It is not very unusual to have an isolated LDH elevation. LDH is extremely non-specific. 101 different things can make LDH go up and this limits its usefulness as a tumor marker. Elevation of LDH on day 1 of cycle one of first-line chemotherapy is associated with a worse prognosis and, in my practice, that is the only time I check LDH levels.

It is more unusual to have pelvic lymphadenopathy and no retroperitoneal adenopathy in a metastatic testis cancer but it can happen. For me, the decision about whether to get a biopsy would be based more on whether the distribution of disease seems typical or not for a testicular germ cell tumor. For instance, if the only evidence of metastatic d...

I agree completely with @Dr. First Last. After initial workup, I rarely obtain serum LDH as it is not a true "marker" like hCG and AFP. Simple things like recent trauma (or surgery), vigorous sports, or, if on chemo, even neulasta can elevate LDH. Finally, in the absence of undescended testis, scrotal biopsy, vasectomy, or inguinal hernia surgery, it is very rare to present with pelvic only adenopathy.