I was tentatively diagnosed with Mastocytosis too for almost a year by a local doctor, despite negative tryptase, normal urine histamine levels and other markers for Mast Cell disorders. I even had a bone marrow biopsy which was negative for any abnormal mast cell activity. I was also tested for a slew of other conditions, including hereditary angioedema, leukemia, heart failure, ischemic heart disease, kidney failure, liver failure, low protein, autoimmune disorders/allergies, infections, etc, which are all conditions that could cause systemic swelling symptoms like mine. All negative. I took cromolyn, antihistamines and two other meds designed to help suppress mast cell activity but none of them worked. Cromolyn seemed to help a little with intestinal issues but not much. I still had systemic edema every day. I always feel very dehydrated, as well, despite the fluid retention and I live on liquids exclusively due to intestinal swelling (my abdomen is usually grossly distended to the point of looking like a preganant woman so have no appetite). On good days, I urinate frequenty, abdomen will deflate and I drop about 10-15 pounds of fluid within a few days, but the whole cycle starts up again day or two later. I am never truly free of edema but it does wax and wane. I'm very disabled from this condition.
Anyway, I went to the Mayo clinic in MN last year and saw a hematologist there who has published on SCLS and he diagnosed me with chronic capillary leak syndrome and recommended a course of IVIG to see if it would work. Depite the fact that I don't exihibit so called markers for the condtion like an M spike and overt hemoconcentration, he still diagnosed me with the condition because my capillaries are leaking and I am third spacing fluid systemically. I am hypovolemic as well (low blood volume) and my pressure drops from normal of 120/80 to the 100/60 range when I am very swollen. Worst reading was 90/50 when I went to the ER years ago for breathing issue, swelling and dehydration.
SLCS is diagnosis of exclusion so if they've exluded all other known causes of third spacing then that's a diganosis you and your doctors should seriously consider pursuing.
I am in contact by phone with two other members of these boards who have been diagnosed with the chronic variant of SLCS and both are on IVIG for it. They were both misdiagnosed for many years and became very ill and disabled from the condtion. One of them almost died from it and has permanent disabilities because his doctors took so long to figure out what going on with him. I myself have recently developed pulmonary issues from years of living with this disorder. I am hypoxic when I lie down at the doctor's office now (oxygen readings in my blood dip very low when I lie down on by back). My immunologist thinks that it's due to the leaks/swelling. She suspects that my distended abdomen is putting too much prerssue on my diaphram and lower lungs restricting my breathing or possibly that fluid is now leaking into my lungs. She referred me to a pulmonologist and I'll know more when I see him in a few weeks. I have also developed abnormal EKG readings which nobody can figure out the cause of (my echos are still normal and an angiogram shows clear arteries). Cardiologist suspects it's from the swelling.
I'm still waiting to get on IVIG to see if it will help me. My insurance is giving me a very hard time about covering costs so my doctor is trying to get in contact with several of the companies that manufacture IVIG to see if they can help out.
At any rate, I hope you get some answers soon. This is not a condtion you want to let fester over time. It will ultimately cause a lot of other medical problems down the road if it's not addressed and managed properly. Also, don't let anyone tell you that chronic capillary leak syndrome doesn't exist. It does. I am not sure if it's being caused by the same underlying pathology as the more acute version of the condition but it does exist. If doctors ever tell you otherwise, it's because they're ignorant of the condtion and unware of the literature published on this disorder. I am living proof that a person's capillaries can be in a state of chronic abnormal leak with no obvious cause. It's a grossly underfunded and under-researched disorder and in my physician's opinion likely a very underdiagnosed one too, due to lack of definitive diagnostic tests and knoweledge about the condition. Don't let yourself fall through the cracks like I did.
Also, BTW, here's a link to synopsis of a published paper about chronic capillary leak syndrome. https://www.ncbi.nlm.nih.gov/pubmed/30431430
It's the first ever published lit review on the chronic form of SCLS. Includes a case report as well. It was published this year. My doctor called the authors of the paper and they sent her a full text version.
Good luck to you,