Bicarbonate

Your metabolism makes acid all day long: when burning protein, during exertion, when fasting. Yet the acidity of your blood barely moves, and that is thanks to bicarbonate. It absorbs acid and then leaves your body as carbon dioxide through the lungs, while your kidneys make new bicarbonate and reclaim it from the pre-urine. Lungs and kidneys therefore turn one dial together: the lungs in minutes, by breathing faster or deeper, the kidneys in hours to days. The value in your blood is the level of that buffer. If it falls, more acid is being offered than is being removed, or the kidneys are holding on to too little bicarbonate. If it rises, acid is being lost or buffer is being added. The number says that the balance has shifted, not why; for that chloride, sodium, potassium and kidney function are needed beside it. Your report shows bicarbonate in mmol/L. Normal is about 24 to 28 mmol/L where the labs overlap; because one lab uses a narrow band and another a wide one, a value just under 24 is not called the same everywhere. It is measured in a separate tube that must stay closed, because bicarbonate escapes as carbon dioxide as soon as air gets to it.

For anyone healthy, bicarbonate is rarely the value that tells something new: the buffer works, and the number sits where it belongs. It becomes interesting in two situations. The first is a kidney function that is declining. Kidneys that filter less also excrete less acid, and bicarbonate then slowly falls with it; it is one of the first values to show that the kidney can no longer get rid of the acid, and a low value in turn speeds up bone loss and muscle breakdown. In chronic kidney disease it is therefore followed and topped up where needed. The second situation is a disturbance of the salt and fluid balance: persistent vomiting and diuretics make the value rise, severe diarrhoea makes it fall, and a ketogenic diet or prolonged fasting lowers it slightly, because ketones are acids. What food does to it is small but measurable. Protein, meat, cheese and grains deliver acid, vegetables and fruit deliver base; anyone who eats a lot of the first and little of the second asks more of the buffer, and at an older age and with weaker kidneys you see that back in a bicarbonate on the low side. More vegetables and fruit is then the lever, and that is the same lever as for blood pressure and bone. And then the most common reason for a low value, which has nothing to do with you: a tube that stood open too long. Bicarbonate disappears as carbon dioxide, and a result lower than expected more often says something about the logistics than about your kidneys.

A low bicarbonate is, in order of how often it occurs: a tube that stood open or waited too long, a kidney function that excretes less acid, severe diarrhoea, a ketogenic diet or prolonged fasting, and rarely a derailment of blood sugar in diabetes or a poisoning. Breathing too fast and deep, in panic or hyperventilation, lowers it too, because the lungs then remove too much carbon dioxide and the kidneys compensate. Chloride beside it helps: if that rises with it, it is usually loss through the gut or the kidneys; if it stays the same, acid has been added somewhere. A high bicarbonate means acid loss or extra buffer: persistent vomiting, diuretics, plenty of antacids or bicarbonate tablets, and in people with a lung disease the kidneys compensating for a chronically high carbon dioxide. In a healthy person it rarely occurs. Set bicarbonate beside chloride, sodium and potassium, and beside creatinine and eGFR. The number says the buffer is under pressure; the values beside it say why.