Can't find answers to what's wrong with cat

Hello Vets   About 6 weeks ago, I noticed one of my cats having trouble eating and seemed in pain. He was taken to the vet and we found his gums very red and teeth falling out with ease. His weight was noticeably less than a previous reading for him. We removed the trouble teeth and he did much better for a while. Last week he seemed to be having trouble again, so we went back to the vet. Gums again very red and more weight loss. We have done additional blood work, which I will post here. Many results are in Greek so I have done my best to translate on image. Felv and FIV both negative. Based on his blood work we did an exploration ultrasound, and everything looks completely normal, they found nothing to indicate a problem. All lymph nodes normal. We drew more blood and it was very dark, almost a black tint. 3 vets consulted on his case and non can think of what the problem is, so we are treating him blind and hoping it helps. He has lost more weight in a week and isn't eating. Treatment includes long lasting cortisone injections and 12 days of broad spectrum antibiotics. At this point we are being told it probably won't help and he will continue to decline. I'm hoping someone here might be able to think of a cause of his blood work that we have not thought of.   Thank you in advance   (For information, he is a 7 year old Korean short hair, fully indoor cat, exposure to any infectious diseases is non existent, weight loss - total 1.6kg - 200g in a week)

Hello it sounds as though currently, the main symptom is that your cat is off their food and losing weight; it sounds as though the vet has carried out considerable much-needed dental work, but that the gums are red and sore and your cat is still off their food.  You also tell me that the blood is almost black, but not why; may the patient be dehydrated, or their red blood cells be being broken down?  Your vet can do tests in order to determine this.   I notice that the pancreatic enzymes are elevated, although this may not be significant; has you vet ruled feline pancreatitis out?  This is a common cause of reduced appetite and weigh-loss.  Finally, what infectious diseases could this still be?  I don't know where you are geographically, but I wondered about Heamobartonella Felis, for example.   I think that a good way forward would be to ask your vet for a list of possible explanations for what is going on here; 'What could still be causing this?'  This will hopefully show that they are tackling the case in a systematic way.  Depending on the kitten's condition, they may have plans to prevent deterioration of his health in addition to ruling out the next most likely possibility.