Introduction

Protein is not normally found in the urine. Elevated level of protein in the urine (proteinuria) can be detected in the majority of kidney diseases, with the increasing amounts of protein in the urine reflecting increasing kidney damage.

Definition

24-hour urine protein measures the amount of protein excreted in urine over a 24-hour period.

Why Do I Need This Test?

The doctor may order this test if you have signs or symptoms of glomerular disease, such as nephrotic syndrome, or other condition that may affect kidney function. Urinary protein can be estimated in random samples, timed or untimed overnight samples, or in a 24-hour specimen. The recommended method for quantitating protein in the urine is a 24-hour collection as the 24 hours period allows for changes in circadian rhythm in excretion for the day.

Procedure For 24-Hour Urine Collection

During a 24-hour urine collection, keep to your usual diet and drink, unless instructed otherwise. Avoid drinking alcohol before and during the urine collection.

The 24-hour urine collection follows this procedure:

1. You will be given one labeled container with or without preservative to store your urine. You will need to transfer the urine from a collecting container to the storage container.
2. The 24-hour collection may begin at any time. However, it is common to start the collection the first thing in the morning.
3. Do not save the urine from your first time urinating. Discard this first specimen, but note the time. This will be the start time of the 24-hour collection.
4. Collect all urine passed during the next 24 hours in the container provided. Mix the contents thoroughly after each addition of urine.
5. Try to urinate again at approximately the same time the following day. This last urine should be included in the total collection.
6. Return the urine container to the laboratory once the urine collection has been completed. If there is a delay in returning the urine sample, the container should be kept cold either by refrigeration or keeping in an ice-bath.

Laboratory Methods For Urine Protein Estimation

Prior to 1990, the methods used to quantitate total protein concentration in urine were mainly precipitation assays with turbidimetric or nephelometric determinations. Dye binding assays were then used by only a minority of laboratories. Today, most laboratories utilize direct dye binding assays or automated turbidimetric assays.

In the frequently used sulfosalicylic acid method, sulfosalicylic acid is added to the urine sample. A photometer or nephelometer is used to measure the turbidity. Total protein is then quantified through comparison of the turbidity of the sample with the turbidity of the standard. Nevertheless, this method lacks precision with a high coefficient of variation. The sulfosalicylic acid / biuret technique has also been shown to be impractical, requires a large sample and is not linear at low concentrations of urine protein. The quantitative Trichloroacetic Acid – biuret method is tedious and has better precision.

Some dye-binding colorimetric methods to measure urinary protein includes Coomassie Blue, Ponceau S, benzethonium chloride and benzalkonium Chloride turbidity methods. Pyrogallol Red-Molybdate will react with protein to form a bluish purple complex which can be measured at 600nm. At present no single method has been widely utilized by laboratories. Although protein dye-binding assays are rapid, simple and readily automated, they lack a uniform response to different proteins.

What Are The Expected Results For This Test?

Normally average daily urinary protein excretion in adults is 80 mg/day, with normal excretion considered to be <0.16 g/day.

As protein has a very low maximal tubular rate of reabsorption, detection of an elevated amount of protein is an important indicator of renal disease. It may be a sign of kidney damage / disease or it may be a temporary increase caused by an infection, medication, vigorous exercise as well as physical or psychological stress. In some people, it may be detected during the day and absent at night when the patient is lying down (postural proteinuria). In pregnant women, elevated urine protein concentration can be related to pre-eclampsia.

When kidney damage is present, the increasing level of protein present in the urine is generally associated with the severity of damage and decreasing kidney function.

Special Conditions

1. You filled up the container provided by the laboratory before the 24-hour period is completed:-
   • Use a very clean glass or plastic container to continue collecting your urine, keeping it chilled.
2. For patients with urinary catheters:-
   • Start your collection with a fresh catheter bag in place. If that is not possible, clean the existing bag (remove the bag from the catheter and rinse it out).
   • Begin your collection by completely emptying the current catheter bag and flushing the accumulated urine. Record this as your start time. During the 24-hour-collection time, empty the bag into the collection container at regular intervals and keep the container chilled. At the designated stop time, empty your collection bag for the last time.
3. Urine contaminated with feces or blood:-
   • Stop collection as the test results may be inaccurate. Return the container to the laboratory. The laboratory will give a new container for you to repeat collection all over again.
4. If you forget to collect all of your urine / If you missed a urine collection during the 24 hour collection period:-
   • Stop collection as the test results may be inaccurate. Return the container to the laboratory. The laboratory will give a new container for you to repeat collection all over again.

References