PBG

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Alternative names
PHB; Porphobilinogen

Definition
This is a test that measures the amount of PBG in urine.

How the test is performed
A 24-hour urine sample is needed. The health care provider will instruct you, if necessary, to discontinue drugs that may interfere with the test.

• On day 1, urinate into the toilet upon arising in the morning.
  
• Collect all subsequent urine (in a special container) for the next 24 hours.
  
• On day 2, urinate into the container in the morning upon arising.
  
• Cap the container. Keep it in the refrigerator or a cool place during the collection period. Label the container with your name, the date, the time of completion, and return it as instructed.

For an infant, thoroughly wash the area around the urethra. Open a Urine collection bag (a plastic bag with an adhesive paper on one end), and place it on your infant. For boys, the entire penis can be placed in the bag and the adhesive attached to the skin. For girls, the bag is placed over the labia. Place a diaper over the infant (bag and all).

The infant should be checked frequently and the bag changed after the infant has urinated into the bag. For active infants, this procedure may take a couple of attempts—lively infants can displace the bag, making it difficult to obtain the specimen. The urine is drained into the container for transport to the laboratory.

Deliver it to the laboratory or your health care provider as soon as possible upon completion.

How to prepare for the test
No special preparation is necessary for this test, but if the collection is being taken from an infant, a couple of extra collection bags may be necessary.

How the test will feel
This test involves only normal urination, and there is no discomfort.

Why the test is performed

This test may be performed when porphyria or another disorder associated with an abnormal porphobilinogen (PBG) level is suspected.

PBG is one link in the chain of chemical reactions the body uses to to make heme. The heme molecule is the oxygen-carrying part of Hemoglobin, which is found in red blood cells.

Various porphyrins exist. All have the same basic structure, but a slightly different chemical appearance. The porphyrin molecule goes through several intermediate chemical stages (delta-ALA, PBG, uroporphyrin, coproporphyrin, and protoporphyrin) to finally yield the end product, heme.

Each step requires the presence of an enzyme. If any of the enzymes are deficient (because of a genetic disease or inhibition by a toxic substance), porphyria results.

Normal Values

Normal Values
are as follows:

• random urine: negative test
  
• 24-hour urine: less than 4 mg (milligrams) per 24 hours

What abnormal results mean

Increased levels of urinary PBG may indicate:

• Liver Cancer
  
• hepatitis
  
• lead poisoning
  
• porphyria (several types)

What the risks are

There are no risks.

Special considerations

Drugs that can affect test measurements include: aminosalicylic acid, barbiturates, chloral hydrate, chlorpropamide, ethyl alcohol, griseofulvin, morphine, oral contraceptives, phenazopyridine, procaine, and sulfonamides.

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