DEAR DR. DONOHUE: I had 1.5 liters (about 1.5 quarts) of fluid withdrawn from my right pleural cavity. Thirty-one days later, I had another 2 liters drained. I asked my doctor what caused it, and he said he didn’t know. I am on a diuretic to remove excess body water. Where does the fluid come from? The first lab results showed atypical cells in the fluid. The second results showed suspicious cells. A biopsy is recommended. Is cancer in the pleural cavity considered lung cancer? — R.S.

ANSWER: The lung is covered by two sheets of cellophanelike material — the pleura. A small space exists between those two sheets — the pleural cavity. A bit of lubricating fluid fills that space to decrease friction as the sheets move across each other with each inhalation and exhalation.

A host of problems can cause fluid to find its way into the pleural cavity. One is infection. The infection can start in the lung and spread into the pleura, or it can start right in the pleural cavity. Inflammation of the cavity causes fluid to drain into the pleural space from the circulation. Congestive heart failure, liver cirrhosis and kidney failure also cause a pleural effusion, as the fluid is called. Again, the fluid comes from an overloaded circulation. Illnesses like rheumatoid arthritis are another cause.

Cancer is the most feared cause. In some instances, the cancer starts out on the pleura. Mesothelioma, an asbestos-caused cancer, is such a malignancy. Or the cancer can spread from the lungs into the pleural cavity, or it can come from a distant site. It is transported to the pleural cavity through the lymph system or the blood. A biopsy of the pleural lining should help solve this mystery. The final answer make take time to come, but the cause will eventually be established.

DEAR DR. DONOHUE: Eighteen months ago, I was diagnosed as having type-2 diabetes. I was put on two oral medicines and insulin. I reduced my weight from 350 pounds to 235. I am 6 feet 4 inches tall. My HbA1c is 5.5. I maintain blood sugar levels of 65 to 95 (3.6 to 5.2) before meals and less than 140 (7.7) after meals. The physician’s assistant reduced my insulin dose. Because of my low readings, I have stopped my insulin and reduced the oral medicines. What should be my target HbA1c? What is considered normal for a nondiabetic? — M.M.

ANSWER: HbA1c (hemoglobin A1c, also called glycated hemoglobin) is on the verge of becoming the universal test for diagnosis of diabetes and the way to follow its control. It gives an indication of blood sugar over the past three or four months. Hemoglobin (Hb) is the stuff inside red blood cells that holds onto oxygen. High blood sugar coats some of it, sort of like glazing a donut. HbA1c of 7 or slightly less is considered the goal for most diabetes patients. HbA1c of 7 corresponds to a blood sugar (plasma glucose) of 154 mg/dl (8.5 mmol/L); 6, to a blood sugar of 126 (6.9); 5 to a blood sugar of 97 (5.4). A nondiabetic should have an HBA1c level between 5 and 5.5.

Your weight loss merits a round of applause. That has contributed mightily to your good readings. You should discuss with your caregivers reductions in your medicines. If you are taking your own blood sugar readings, then you can make reductions, but you should report them all the same.

DEAR DR. DONOHUE: I am 55 and have osteoporosis. Fosamax hasn’t helped me. My doctor recommends I take Forteo. I read a warning stating that rats treated with Forteo have an increase in the incidence of osteosarcoma dependent on dose and duration. The dose was three to 60 times that used in humans. Should I be afraid to take it? — R.B.

ANSWER: You use the words contained in the manufacturer’s warning. Osteosarcoma is a bone cancer. Since approval by the FDA in 2002, more than 2 million prescriptions for Forteo have been filled. I haven’t found any reported instances of human bone cancer in users. If my doctor advised me to take it, I wouldn’t hesitate for a minute to do so.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.