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Dear Dr. Roach: I am a female in my mid-60s and recently have been diagnosed with a cyst on my left kidney. I do not have diabetes nor a heart condition, never smoked nor used drugs, don’t drink and take over-the-counter or prescription medicines only when absolutely necessary, which is rare. I also don’t like, thus never eat, spicy foods, nor do I drink coffee/tea/caffeine. I have only an occasional candy bar.

I know that kidneys filter out bacteria, but am still wondering how could a cyst, or any renal ailment, be bestowed on such “clean” kidneys? Can you please also elaborate on kidney cysts?

V.B.

Dear V.B.: A cyst is just a fluid-filled sac, and they are common in the kidneys, especially as people get older. They are more common in men: Among adults aged 50-70, 15 percent of men and 7 percent of women have had a kidney cyst.

It does not seem that diet or medication predispose someone to developing kidney cysts. They may arise from diverticula (outpouchings) in the kidney’s collecting tubule. Obstruction of the tubule, such as by a stone, may predispose to them, too, but it is certainly not necessary for a stone to develop a kidney cyst.

Most simple cysts do not require treatment. Once it is clear that the cyst is not cancerous (rare), it can be left alone, or sometimes may require a follow-up scan to be sure the cyst is stable.

Incidentally, the kidneys shouldn’t have to filter out bacteria. The kidneys filter small- and medium-sized waste products from the blood and help regulate salt and water balance. Bacteria shouldn’t be in the blood for more than a few minutes (bacteria sometimes come through the body’s defenses, especially in the gastrointestinal tract, from the mouth all the way to the end): There’s no reason you should have bacteria in your blood.

Email questions to ToYourGoodHealth@med.cornell.edu.

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