What is Protein C test? 

The protein C test determines the amount of the glycoprotein (a protein with an attached carbohydrate group), protein C, in your blood.

Protein C prevents excessive clotting of blood in your body during an injury.

It circulates in the plasma and is activated during an injury to form activated protein C (APC). This active form acts as an anticoagulant, which inactivates coagulation factors V and VIII.

If you have a protein C deficiency, your blood may clot too much and may block a blood vessel. The clot can form in any part of the body and travel to your lungs. If a clot reaches your lungs, it can cause a life-threatening condition called pulmonary embolism.

If your protein C levels are higher than normal, it will not cause any medical problem.

Protein C deficiency is an inherited disorder. Rarely, the deficiency may be acquired later in life due to other causes. The deficiency may be mild or severe. People with mild symptoms may never have any life-threatening blood clots. However, the risk of developing serious clots increases in the following conditions:

The protein C test may help determine ways to prevent clot formation, regardless of the deficiency being inherited or acquired.

  1. Why is Protein C test performed?
  2. How do you prepare for a Protein C test?
  3. How is a Protein C test performed?
  4. Protein C test results and normal range

Doctors generally order this test in case of a thrombotic (clotting) episode, like venous thromboembolism. The test may especially be ordered in the following situations:

  • Blood clot in a person below 50 years of age
  • Unexplained blood clot 
  • Blood clots in unusual locations, such as in the blood vessels of the kidney, liver or the brain
  • Newborn with a suspected severe clotting disorder, such as purpura fulminans or disseminated intravascular coagulation
  • Family history of protein C deficiency
  • Family history of clotting disorders

The protein C test is also used as a screening test in relatives of people with protein C deficiency as it is an inherited disorder. 

The test may also be performed to determine the cause of multiple miscarriages in women.

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You do not need to fast for the test. Inform your healthcare provider about all the prescription and over-the-counter drugs as well as vitamins and supplements you may be taking. Your doctor may ask you to discontinue a few drugs that can affect the test results. These drugs are:

The test should be performed after at least 10 days have elapsed since the clotting episode.

Protein C test is performed on a blood sample. A laboratory technician will draw the sample from a vein in the arm in the following manner:

  • He/she will tie a tight band, called a tourniquet, around your upper arm and clean the site of injection with an antiseptic. 
  • With the aid of a needle and a syringe, the technician will draw the blood. 
  • Once the sample is obtained, the technician will remove the tourniquet and place a small swab of cotton on the injection site. 
  • He/she will transfer the sample into a labelled container and send it to the laboratory for testing.

There are a few risks associated with a blood test. These include:

You may feel slight pain when the needle is pricked, but it will subside soon.

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Normal results:

The normal protein C levels in the blood are 70%-150% of normal activity. The levels of protein C decrease with age. Females have a lower protein C level compared to males.

The tests results may vary depending on the following:

  • Gender
  • Age
  • Medical history 
  • The method used to perform the test 

Discuss your results with your doctor to know what they mean.

Abnormal results:

Increased levels are not clinically relevant.

Decreased levels of protein C in the blood may be seen in the following conditions:

Blood thinners like warfarin may also cause low protein C levels.

Your doctor may repeat the protein C test before confirming the diagnosis. In case of an acquired deficiency, the doctor may occasionally monitor protein C levels as the condition resolves or progresses.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

References

  1. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; Protein C (Blood)
  2. Pabinger I, Allaart CF, Hermans J, et al. Hereditary protein C-deficiency: laboratory values in transmitters and guidelines for the diagnostic procedure. Report on a study of the SSC Subcommittee on Protein C and Protein S. Protein C Transmitter Study Group. Thromb Haemost 1992;68:470-474. PMID: 1448781.
  3. Marlar RA, Montgomery RR, Broekmans AW. Diagnosis and treatment of homozygous protein C deficiency. Report of the Working Party on Homozygous Protein C Deficiency of the Subcommittee on Protein C and Protein S, International Committee on Thrombosis and Haemostasis. J Pediatr. 1989;114(4 Pt 1):528–534. PMID: 2647943.
  4. Mannucci PM, Owen WG: Basic and clinical aspects of proteins C and S. In: AL Bloom, DP Thomas. Haemostasis and Thrombosis. Second edition. Edinburgh, Churchill Livingstone, 1987, pp 452-464
  5. Merck Manual Consumer Version [Internet]. Kenilworth (NJ): Merck & Co. Inc.; c2018. Excessive clotting
  6. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Blood Tests
  7. National Organisation of Rare Disorders [Internet]. Danbury. CT. US; Protein C Deficiency
  8. Genetics Home Reference [internet]. National Institute of Health: US National Library of Medicine. US Department of Health and Human Services; Protein S deficiency
  9. UFHealth [internet]: University of Florida; Protein C blood test. Gainesville. Florida. US; Protein C blood test
  10. UW Health: American Family Children's Hospital [Internet]. Madison (WI): University of Wisconsin Hospitals and Clinics Authority; Deep Vein Thrombosis
  11. Mosnier LO, Zlokovic BV, Griffin JH. The cytoprotective protein C pathway. Blood. 2007;109(8):3161–3172. PMID: 17110453.
  12. Esmon CT. The protein C pathway. Chest. 2003;124(3 Suppl):26S–32S. PMID: 12970121.
  13. Bengtsson C, Lindquist O. Characteristics and prognosis for women with hypertension in a community: results of a longitudinal study in Göteborg, Sweden. Clin Sci Mol Med Suppl. 1976;3:649s–651s. PMID: 1071699.
  14. Tabashnik BE. Determining the mode of inheritance of pesticide resistance with backcross experiments. J Econ Entomol. 1991;84(3):703–712. PMID: 1885840.
  15. Beckmann RJ, Schmidt RJ, Santerre RF, Plutzky J, Crabtree GR, Long GL. The structure and evolution of a 461 amino acid human protein C precursor and its messenger RNA, based upon the DNA sequence of cloned human liver cDNAs. Nucleic Acids Res. 1985;13(14):5233–5247. PMID: 2991859.
  16. Foster DC, Yoshitake S, Davie EW. The nucleotide sequence of the gene for human protein C. Proc Natl Acad Sci U S A. 1985 Jul; 82(14): 4673–4677. PMID: 2991887.
  17. Mather T, Oganessyan V, Hof P, Huber R, Foundling S, Esmon C, et al. The 2.8 A crystal structure of Gla-domainless activated protein C. EMBO J. 1996 Dec 16; 15(24): 6822–6831. PMID: 9003757.
  18. Hézard N, Bouaziz-Borgi L, Remy MG, Florent B, Nguyen P. Protein C deficiency screening using a thrombin-generation assay. Thromb Haemost. 2007;97(1):165–166. PMID: 17200789.
  19. Pagana KD, et al. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Mosby. Protein C, Protein S. Pg. 751,752
  20. Wilson DD. McGraw-Hill’s Manual of Laboratory & Diagnostic Tests. 2008. McGraw Hill. Protein C. Pg 470-471.
  21. Fischbach FT. A Manual of Laboratory and Diagnostic Tests. 7th ed. 2003. Lippincott Williams & Wilkins Publishers. Protein C. Pg. 110-111.
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