Urine protein creatinine ratio (PCR) explained

Healthy kidneys act as a sieve, keeping most protein in the blood. Lab Tests Online UK explains that proteins are not usually found in urine, and when the kidneys are damaged, detectable amounts leak through. Often the smaller albumin molecules are detected first, and if damage continues, the amount increases and globulins may also be lost.

Creatinine is a by-product of muscle metabolism that is normally excreted into urine at a constant rate. Measuring creatinine in the same sample as the protein therefore corrects for how dilute or concentrated the urine is. The result, the protein-to-creatinine ratio, is reported in milligrams of protein per millimole of creatinine (mg/mmol) and can be used instead of a 24-hour urine protein test, which is cumbersome to collect and only accurate if every drop is saved.

Both tests use a spot urine sample and both divide by creatinine. The difference is what is measured in the numerator. ACR measures albumin specifically and PCR measures total protein, including albumin, globulins and any abnormal proteins such as light chains in myeloma.

NICE's chronic kidney disease guideline recommends using urine ACR rather than PCR for the initial detection of proteinuria in adults, children and young people because of its greater sensitivity for low levels. A confirmed ACR of 3 mg/mmol or more is regarded as clinically important proteinuria. NICE says that when ACR is 70 mg/mmol or more, PCR can be used as an alternative. It also advises against using reagent strips for adults unless they specifically measure albumin and express the result as an ACR.

Lab Tests Online UK says a protein-to-creatinine ratio may be requested on a random urine sample when a child shows evidence of significant and persistent protein on a dipstick, and either a 24-hour protein or a random ratio may be used to monitor someone with known kidney disease. It may also be used as a regular screen for kidney involvement when someone takes a medicine that may affect kidney function.

It can also be requested when a clinician suspects proteins other than albumin are being lost, since the dipstick primarily measures albumin and can be normal when significant amounts of other proteins are present. In that situation, urine and serum electrophoresis may be added, especially if abnormal protein production such as multiple myeloma is suspected.

NICE's guideline on hypertension in pregnancy says that if dipstick screening is positive (1+ or more), an albumin:creatinine ratio or protein:creatinine ratio should be used to quantify proteinuria. It recommends not using the first morning void and not routinely using a 24-hour collection.

If using PCR, NICE says to use 30 mg/mmol as the threshold for significant proteinuria, and if the result is 30 mg/mmol or above and there is still uncertainty about the diagnosis of pre-eclampsia, consider re-testing on a new sample alongside clinical review. The definition of pre-eclampsia in the same guideline combines new-onset hypertension after 20 weeks with proteinuria (PCR 30 mg/mmol or more, ACR 8 mg/mmol or more, or 2+ on dipstick) or other organ dysfunction. Lab Tests Online UK also notes that elevated urine protein in pregnancy can be associated with pre-eclampsia.

Lab Tests Online UK calls protein in the urine a warning sign, which may indicate kidney damage or disease or be transient because of infection, medication, vigorous exercise or physical stress. In some people it is present during the day and absent at night; orthostatic proteinuria, which disappears on lying down, is commonly seen with increasing age and carries no health risk. When kidney damage is present, the amount of protein is generally associated with the severity of damage, and rising amounts over time indicate increasing damage.

The test does not say which protein is present or the cause. Conditions associated with proteinuria include diabetes, high blood pressure, glomerulonephritis, polycystic kidney disease, lupus, amyloidosis, heart failure, myeloma, urinary tract infection and some medicines. Because transient causes are common, a single raised result is usually repeated before conclusions are drawn.

No preparation is needed; a random urine sample is collected in a clean container. For 24-hour collections, all urine for the period must be saved and kept refrigerated. Try to collect the sample as instructed, because the timing guidance differs between conditions, for example the pregnancy advice not to use the first morning void.

Ask which ratio your clinician is using and what threshold applies in your situation, since NICE uses different cut-offs for kidney disease and for pregnancy, and the numbers on reports from different laboratories may not be directly comparable.