Zinc protoporphyrin (ZPP) is primarily ordered to help detect iron deficiency in children and to detect and monitor chronic exposure to lead in adults.
ZPP is a substance that is normally found in small amounts in red blood cells. Most of the protoporphyrin in red blood cells combines with iron to form heme, the molecule in hemoglobin that carries oxygen. Zinc combines with protoporphyrin instead of iron when there is insufficient iron available to form heme, as in iron deficiency, or when lead is present and blocks the formation of heme, as in lead poisoning. The level of ZPP in the blood will rise with these conditions.
Two types of test are available to measure ZPP:
The free erythrocyte protoporphyrin (FEP) test measures both ZPP, which accounts for 90% of protoporphyrin in red blood cells, and free protoporphyrin, which is not bound to zinc.
The ZPP/heme ratio gives the proportion of ZPP compared to the normal iron-containing heme in red blood cells.
ZPP testing may be ordered, along with a lead level, to test for chronic lead exposure. Hobbyists who work with products containing lead and people who live in older houses may be at an increased risk of developing lead poisoning. Those who inhale dust that contains lead or handle lead directly and contaminate their hands and then eat may have elevated lead levels. Children who eat paint chips that contain lead (common in houses built prior to 1960) can have elevated levels of lead and ZPP in their body.
In an industrial setting, the Occupational Safety & Health Administration (OSHA) mandates the use of the ZPP test and strongly recommends that a ZPP test be ordered every time that a lead level is ordered to monitor an employee's exposure to lead. Both are necessary because ZPP will not reflect recent or acute lead exposure and it does not change quickly when a person's source of lead exposure is removed. ZPP is best at detecting a person's average exposure to lead over the last 3-4 months.
ZPP is not sensitive enough for use as a lead screening test in children, as values do not rise until lead concentrations exceed the acceptable range. The maximum lead concentration considered safe in children has been set at a very low level by the Centers for Disease Control and Prevention (CDC). In this age group, tests that measure the blood lead concentration are done to detect exposure to lead.
Iron deficiency anemia
In children, the ZPP/heme ratio is sometimes ordered as an early indicator of iron deficiency. An increase in the ZPP/heme ratio is one of the first signs of insufficient iron stores and will be elevated in most young people before signs or symptoms of anemia are present. More specific tests of iron status are required to confirm iron deficiency.
The test may be ordered when an employee is a participant in an occupational lead monitoring program or when someone has a hobby, such as working with stained glass, that brings the person into frequent contact with lead.
The ZPP/heme ratio may be ordered as a screening test for iron deficiency in children and adolescents or when iron deficiency is suspected. Some symptoms of iron deficiency include:
Chronic fatigue, tiredness
If the iron-deficiency anemia is severe, shortness of breath, dizziness, chest pain, headaches, and leg pains may occur. Children may develop learning (cognitive) disabilities. Besides the general symptoms of anemia, there are certain symptoms that are characteristic of iron deficiency. These include pica (cravings for specific substances, such as licorice, chalk, dirt, or clay), a burning sensation in the tongue or a smooth tongue, sores at the corners of the mouth, and spoon-shaped fingernails and toenails.
The ZPP concentration in blood is usually very low. An increase in ZPP indicates a disruption of normal heme production but is not specific as to its cause. The main reasons for increases in ZPP are iron deficiency and lead poisoning.
It is important that ZPP levels be evaluated in the context of a person's history, clinical findings, and the results of other tests such as ferritin, lead, and a complete blood count (CBC). It is possible that the person may have both iron deficiency and lead poisoning.
In cases of chronic lead exposure, ZPP reflects the average lead level over the previous 3-4 months. However, the amount of lead currently present in the blood and the amount in the organs and bones cannot be determined with a ZPP test. Values for ZPP rise more slowly than blood lead concentrations following exposure, and they take longer to drop after exposure to lead has ceased.
An increase in the ZPP/heme ratio in a child is most often due to iron deficiency. A decreasing ZPP/heme ratio over time following iron supplementation likely indicates successful treatment.
ZPP may be elevated in inflammatory conditions, infections, and in people with anemia of chronic disease, but it is not generally used to monitor or diagnose these conditions. A very high ZPP level may be seen in two hereditary diseases: protoporphyria and congenital erythropoietic porphyria (see Porphyria). Both of these disorders are very rare and present with symptoms related to the skin (extreme sensitivity to sunlight).
Depending on the method used to test ZPP, high levels of other substances in the blood, such as bilirubin and riboflavin, can produce false-positive results. Falsely low values may occur if the sample is not protected from light before testing.
This article was last reviewed on September 18, 2014. | This article was last modified on September 18, 2014.
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