Test Details
Methodology
Protein C in plasma is activated by a specific enzyme (protein C activator) from copperhead snake venom (Agkistrodon contortrix). The amount of activated protein C is determined. The resulting activated PC (Protein Ca) is determined as the rate of hydrolysis of a chromogenic substrate in a kinetic test by measuring the increase in absorbance at 405 nm.
Result Turnaround Time
2 - 3 days
Turnaround time is defined as the usual number of days from the date of pickup of a specimen for testing to when the result is released to the ordering provider. In some cases, additional time should be allowed for additional confirmatory or additional reflex tests. Testing schedules may vary.
Use
This test is used for confirmation and characterization of protein C deficiency.
Special Instructions
If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions.
Limitations
Individuals with heterozygous PC deficiency may have low normal PC levels.6 Treatment with warfarin decreases the levels of vitamin K-dependent factors including PC. PC levels start to drop after six hours of warfarin treatment and do not regain pretreatment levels until generally two weeks after cessation of therapy. Elevated factor VIII levels, as can be seen in acute phase reaction, can normalize the aPTT and effectively reduce PC levels.7 PC levels can be falsely low in patients with the factor VLeiden mutation. PC levels can become depleted as the result of activation of coagulation limiting the utility of testing for congenital PC deficiency during the immediate convalescent period after a thrombotic event.6 Heparin therapy up to 1 unit/mL does not affect PC levels.6 This test should not be used for patients receiving thrombin inhibitors such as hirudin and argatroban.7
Custom Additional Information
See Protein C Deficiency Profile [283655] for more clinical information.
Specimen Requirements
Specimen
Plasma, frozen
Volume
1 mL
Container
Blue-top (sodium citrate) tube
Collection Instructions
Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood-to-anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when the sample is collected using a winged (i.e., "butterfly") collection system. With a winged blood collection set a discard tube should be drawn first to account for the dead space of the tubing and prevent under-filling of the evacuated tube.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternative anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes.
Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.
Stability Requirements
| Temperature | Period |
|---|---|
| Frozen | 28 days |
| Freeze/thaw cycles | Stable x3 |
Reference Range
| Age | Range |
| 1 d | 24–40% |
| 3 d | 24–51% |
| 1 to 11 m | 28–124% |
| 1 to 5 y | 50–134% |
| 6 to 10 y | 64–125% |
| 11 to 16 y | 59–112% |
| >16 y | 77–150% |
Storage Instructions
Freeze.
Patient Preparation
Ideally, the patient should not be on anticoagulant therapy. Avoid warfarin (Coumadin®) therapy for two weeks prior to the test and heparin, direct Xa and thrombin inhibitor therapies for about three days prior to testing. Do not draw from an arm with a heparin lock or heparinized catheter.
Causes for Rejection
Severe hemolysis; improper labeling; clotted specimen; specimen diluted with IV fluids; samples thawed in transit; improper sample type; sample out of stability
References
Gupta A, Patibandla S. Protein C Deficiency. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan. 2023 Jul 4. PubMed 31194379
Moore GW. Thrombophilia Screening: Not So Straightforward. Semin Thromb Hemost. 2024 Nov;50(8):1131-1152. PubMed 38733983
STA® – Staclot® Protein C Instructions For Use (IFU) [package insert]. April 2019.
Footnotes
9. Labcorp internal validation of adult reference range for protein C activity.
LOINC® Map
| Order Code | Order Code Name | Order Loinc | Result Code | Result Code Name | UofM | Result LOINC |
|---|---|---|---|---|---|---|
| 117705 | Protein C-Functional | 27818-4 | 117705 | Protein C-Functional | % | 27818-4 |
| Order Code | 117705 | |||||
| Order Code Name | Protein C-Functional | |||||
| Order Loinc | 27818-4 | |||||
| Result Code | 117705 | |||||
| Result Code Name | Protein C-Functional | |||||
| UofM | % | |||||
| Result LOINC | 27818-4 |