Son with biliary atresia had an u/s. 3 weeks post stomach flu; pre cold. Showed some enlargement of abdominal nodes. Doc wants ldh blood test and u/s in 6 wks. Wouldn't it show if they were reactive?

Depends. Ultrasound can show enlarged lymph nodes but cannot qualitatively determine cause or if reactive versus other cause.
Lymph nodes. Lymph nodes vary in size with illness and also for not discernable reason whatever. Ldh is old-fashioned for liver but i trust other liver function tests are being assayed as well. Glad junior's being treated; ultrasounds and lab work aren't going to do any harm. Best wishes.