Urine pH: acidity, diet, infection and kidney stones

pH is a scale from 0 to 14 describing acidity: below 7 is acidic, above 7 is alkaline. The kidneys help control the body's acid-base balance by removing acid and reclaiming bicarbonate, and urine pH reflects how much acid the kidneys are excreting at that moment. GPnotebook says the pH of normal urine is generally in the range 4.6 to 8, a typical average being around 6.0, with much of the variation due to diet.

It is one of the chemical tests on a standard reagent strip, which Lab Tests Online UK says are done as part of urinalysis in clinics with results available immediately.

GPnotebook explains that high-protein diets result in more acidic urine, whereas vegetarian diets generally result in more alkaline urine, both within the typical range. Medicines and recent illness can also shift pH, so tell the clinician about both.

Because of this variability, a single value rarely means much. Doctors look at whether pH is persistently high or low and at the other dipstick findings.

Alkaline urine is associated with infection by urease-splitting organisms such as Proteus and Klebsiella. These bacteria split urea into ammonium, raising urine pH to neutral or alkaline values, and are related to the formation of struvite-apatite stones. GPnotebook says struvite stones are potentiated by bacterial infection and that upper urinary tract stones involving the renal pelvis and at least two calyces are called staghorn calculi, of which about 75 per cent are composed of struvite-carbonate-apatite.

The NHS lists symptoms of urinary infection including burning, frequency, cloudy urine, blood, lower tummy or back pain and fever, and describes kidney stones as causing bad pain in the side, lower back or groin that comes and goes with nausea. Persistent alkaline urine with recurrent infections or stone symptoms warrants further investigation.

Urine pH generally reflects blood pH, but not in renal tubular acidosis (RTA). In type 1 (distal) RTA, GPnotebook says the urine is acidic while the blood is alkaline; in type 2 (proximal) RTA the urine is initially alkaline but becomes more acidic as the disease progresses. These are rare conditions diagnosed with blood tests and specialist assessment, and they are among the causes of a normal anion gap acidosis described by Lab Tests Online UK.

A fresh sample gives the most reliable reading. A first morning sample is more concentrated and is most valuable for dipstick screening, according to Lab Tests Online UK, though pH can be tested at any time. Home strips for urine pH, glucose and ketones are available at pharmacies, but the source says these are best performed by someone used to reading and interpreting them.

If pH is abnormal with symptoms, the clinician may send urine for culture, request kidney function and bicarbonate tests, and consider imaging for stones. Drink enough to pass pale urine, and seek urgent help for severe flank pain with fever, vomiting or inability to pass urine. Do not take supplements to change urine pH without advice.