Urine protein (dipstick, ACR and PCR): reading proteinuria and what follows

The kidneys work like a sieve: they filter waste out of the blood into urine while keeping proteins in. Lab Tests Online UK explains that when kidney function is normal, filtered protein is retained or reabsorbed and returned to the blood, and that proteins are not usually found in urine. If the filters are damaged, the smaller albumin molecules leak first, and as damage continues the amount of protein increases and globulins may also be lost.

Proteinuria is therefore an indicator of kidney damage, although small amounts can be present in healthy people, for example with exercise or dehydration. With early kidney damage there are often no symptoms; as protein loss grows, swelling, breathlessness, nausea and fatigue may appear.

A protein dipstick is performed whenever a urinalysis is done, for example in a routine check-up, in pregnancy, when a UTI is suspected or on hospital admission. GPnotebook says chemical strips detect urinary protein at concentrations above 200 mg/L, react specifically with albumin and do not detect Bence-Jones proteins that indicate paraproteinuria such as multiple myeloma. A false positive can be caused by blood in the urine and a false negative by urine that is very acid or very dilute.

Results are graded from negative through trace and plus signs, and the dipstick gives only a rough estimate of whether there is none, a little or a lot. Lab Tests Online UK adds that low levels of protein are better detected in a concentrated specimen collected first thing in the morning.

If slight to moderate protein is detected, the dipstick is usually repeated later to see whether it persists. If there is a large amount or it persists, a laboratory measurement follows. The amount of protein in a 24-hour urine can be measured, but saving all urine is cumbersome, so a random urine can be used instead, reported as the protein to creatinine ratio (PCR). Creatinine is excreted at a steady rate, so the ratio corrects for how dilute the urine is.

ACR measures albumin specifically and is used to screen for early kidney damage in people with diabetes or high blood pressure. Lab Tests Online UK says ACR on a random urine sample is just as good as albumin in a 24-hour collection, and the NHS says kidney disease can be staged using blood and urine tests. Because the dipstick mainly measures albumin, the laboratory test may be requested if other proteins are suspected, with urine and serum electrophoresis to identify which proteins are being excreted, especially if abnormal production such as multiple myeloma is suspected.

Lab Tests Online UK says proteinuria is frequently seen in chronic diseases such as diabetes and high blood pressure, can be present in urinary tract infections alongside other findings, and occasionally results from excess protein production as in multiple myeloma. Children and adults can have transient proteinuria without kidney problems, with higher excretion during the day than at night, so doctors monitor at intervals.

The NHS lists high blood pressure, diabetes, high cholesterol, kidney infections, glomerulonephritis, polycystic kidney disease, blockage of urine flow and long-term use of lithium or anti-inflammatory painkillers among the causes of chronic kidney disease, and says CKD only progresses to kidney failure in about 2 in 100 people with the condition.

No preparation is needed for a random sample. For a 24-hour collection, all urine is collected over a full day and kept refrigerated, with no preservative in the container. Avoid vigorous exercise before the test, and tell staff about fever, infection or periods, which can affect results.

Persistent protein leads to blood tests (creatinine, eGFR, albumin), blood pressure checks, possibly imaging and kidney referral. Treatments protect the kidneys: tight blood pressure and glucose control and medicines that reduce protein leakage. See a GP for swollen ankles, foamy urine, breathlessness or blood in urine.