Protein S Antigen (Free) Citrated Plasma Test: Booking, Price, and Results
About Protein S Antigen (Free) Citrated Plasma Test: Booking, Price, and Results
| Field | Value |
|---|---|
| Also Known As | Free Protein S, Free Protein S Antigen, PS Ag Free, Free PS Ag |
| Sample Type | Citrated plasma (venous blood collected in a light blue-top tube with 3.2% sodium citrate) |
| Fasting Required | No fasting required |
| Report Time | 3 to 5 days |
| Recommended For | Adults (males and females); age- and sex-specific reference ranges apply |
| Price | Starting at ₹5,670 |
What is a Protein S Antigen (Free) Citrated Plasma Test?
The Free Protein S Antigen test measures the level of the biologically active, unbound form of Protein S in your blood. Protein S is a natural anticoagulant protein that helps your body prevent unwanted blood clots from forming. Doctors order this test when they suspect a condition called Protein S deficiency, which can increase the risk of abnormal clotting. It is also known as Free PS Ag or PS Ag Free, and the sample used is citrated plasma drawn from a vein in your arm.
What Does a Protein S Antigen (Free) Citrated Plasma Test Measure?
Protein S circulates in the blood in two forms: a free (unbound) form and a bound form that is attached to a carrier protein called C4b-binding protein. Only the free form is biologically active. This test focuses specifically on measuring that free, active portion.
Here is what the test analyses:
| Parameter | Description |
|---|---|
| Free Protein S Antigen | Measures the unbound form of Protein S responsible for anticoagulant activity |
| Anticoagulant Function Indicator | Helps activated Protein C control blood clotting by regulating clotting factors V and VIII |
| Difference from Total Protein S | Free Protein S measures only the active unbound form, while total Protein S measures both bound and unbound forms |
Why is a Protein S Antigen (Free) Citrated Plasma Test Done?
This test is ordered when there are signs of abnormal blood clotting or a family history of clotting disorders. Below are the main reasons a doctor may prescribe it.
Common Symptoms That May Require This Test
A doctor may request this test if you are experiencing any of the following:
- Unexplained blood clot before 50 years of age
- A blood clot in an unusual location, such as the veins of your kidney, liver, or brain
- Chest pain, especially if accompanied by breathing difficulty
- Fainting or unexplained loss of consciousness (syncope)
- Rapid or irregular heartbeat (cardiac palpitations)
- Recurrent miscarriages
Conditions This Test Can Help Detect
This test helps identify a range of conditions related to abnormal clotting. These include:
- Protein S deficiency (Types I, II, and III), a rare inherited or acquired disorder where Protein S activity is reduced
- Deep vein thrombosis (DVT), where a clot forms in a deep vein, commonly in the legs or arms
- Pulmonary embolism (PE), which occurs when a clot from a vein travels to the lungs
- Pregnancy complications in women with known thrombophilia (clotting disorders), including pre-eclampsia, placental abruption, and stillbirth
- Acquired deficiency states linked to liver disease, nephrotic syndrome, or vitamin K deficiency
How to Prepare and What to Expect
Preparing for this test is straightforward, but a few precautions help ensure your results are accurate.
Do You Need to Fast?
No fasting is required before this test. You can eat and drink normally on the day of collection. However, certain medications can significantly affect the result and should be discussed with your doctor before testing.
Practical Tips Before Your Test
Keep the following points in mind before giving your sample:
- Avoid warfarin therapy for approximately 2 weeks before the test, and heparin therapy for at least 2 days, if your doctor considers this safe for you
- Inform your doctor or the collection staff about all medications and supplements you are currently taking
- Avoid testing during an acute illness, an active clotting event, or immediately after surgery
- Defer this test if you are pregnant or currently on oestrogen-containing contraceptives, as these lower Free Protein S levels naturally
Step-by-Step Procedure
- A tourniquet (a light band) is wrapped around your upper arm to help make the veins more visible.
- The collection site on your arm is cleaned with an antiseptic solution.
- Blood is drawn into a light blue-top tube that contains a preservative called 3.2% buffered sodium citrate; the tube is gently inverted several times to mix the sample.
- The tube is centrifuged twice to separate the plasma from the blood cells.
- The resulting plasma is frozen quickly, within 4 hours of collection, to preserve the sample for accurate analysis.
- The frozen sample is dispatched to the laboratory for testing.
Factors That Can Affect Accuracy
Several factors may interfere with your test result. These include:
- Warfarin use, which reduces Free Protein S Antigen levels due to its vitamin K-blocking effect
- Acute illness or inflammation, which raises levels of the carrier protein and reduces free Protein S
- Pregnancy and oral contraceptive use, both of which cause a natural decrease in free Protein S
- Underfilled collection tubes, which alter the ratio of blood to preservative and produce unreliable results
- High haematocrit (red blood cell concentration above 55%), which requires adjustment of the preservative volume
- Presence of certain antibodies, such as rheumatoid factor, which may cause the test to overestimate levels
Understanding Your Protein S Antigen Free Citrated Plasma Test Results
Your results should always be reviewed together with your doctor, who will consider your age, sex, health history, and other test findings before drawing any conclusions.
The table below shows the reference ranges for different age and sex groups:
| Group | Normal Range (%) |
|---|---|
| Adult Males | 74 to 147 |
| Adult Females | 55 to 123 |
| Females under 50 on hormonal therapy | 50 to 150 |
| Neonates (1 to 89 days) | 15 to 55 |
| Infants (90 to 179 days) | 35 to 92 |
| Infants (180 to 364 days) | 45 to 115 |
| Children (1 to 5 years) | 62 to 120 |
| Children and adolescents (6 to 17 years) | 60 to 140 |
These ranges are general guidelines. Your doctor will interpret your results based on your age, health history, and other factors. Always consult a qualified healthcare professional for personalised medical advice.
Results During Special Conditions
Certain situations can affect how your Free Protein S results should be read:
- Free Protein S levels fall naturally as pregnancy progresses. A low result during pregnancy is likely acquired rather than inherited, so testing for congenital deficiency should be deferred until after delivery and the postpartum period.
- When the body is fighting an infection or inflammatory condition, C4b-binding protein levels rise, which reduces the free fraction of Protein S. This can mimic a deficiency.
- Women generally have lower Protein S levels than men, and premenopausal women on oral contraceptives may have further reduced levels. Children and infants have naturally lower values.
- Protein S may be absorbed by sickle-shaped red blood cells, causing levels to fall.
How to Maintain Healthy Levels
These general wellness habits support liver function and healthy blood flow, which in turn help the body maintain adequate Protein S production:
- Include vitamin K-rich foods in your diet, such as spinach, kale, and broccoli, as Protein S is vitamin K-dependent.
- Avoid smoking and limit alcohol intake to support liver health, since Protein S is produced in the liver.
- Stay well-hydrated and keep up regular physical activity to support healthy circulation.
Lupin Diagnostics Protein S Antigen Free Citrated Plasma Test Price and Home Collection
The Free Protein S Antigen test is available at Lupin Diagnostics with home sample collection across major Indian cities. Approximate prices by city are listed below.
| City | Approximate Price (₹) |
|---|---|
| Mumbai | ₹5,670 |
| Bengaluru | ₹5,670 |
| Chennai | ₹5,670 |
| Hyderabad | ₹5,670 |
| Indore | ₹5,670 |
Prices are indicative and may vary by location. Please confirm the current price at the time of booking.
How to Book
- Select the test on the Lupin Diagnostics website.
- Choose your city and preferred time slot.
- Opt for home sample collection by a certified phlebotomist, or visit your nearest Lupin Diagnostics centre.
- Receive your report via email or WhatsApp within the stipulated turnaround time.
Home Collection
Lupin Diagnostics offers home collection for the PS Ag Free test across cities, so you do not need to visit a centre. All samples are processed in NABL-accredited laboratories by trained staff. Your digital report is delivered to you within 3 to 5 days of collection.
Frequently Asked Questions
The Free Protein S Antigen test measures only the unbound, biologically active form of Protein S. A total Protein S test measures both the free and bound fractions together. Since only the free form carries out anticoagulant activity, it is the more clinically meaningful result when testing for deficiency.
Current guidelines recommend the Free Protein S Antigen assay as the starting point when testing for congenital Protein S deficiency. If the free antigen level falls within the reference range, further Protein S testing is generally not needed.
Warfarin reduces Free Protein S levels because it blocks vitamin K, which the body needs to produce Protein S. It is generally advisable to stop warfarin for 2 to 4 weeks before testing, but only do so with your doctor's approval. Never stop any medication without medical guidance.
Doctors typically order this test when there is a strong family history of venous clotting, recurrent blood clots, an unexplained clot before the age of 50, or a clot in an unusual location such as the brain, liver, or intestines.
Yes, Protein S levels fall naturally as pregnancy advances. For this reason, testing for an inherited Free Protein S deficiency is deferred until after delivery and the postpartum period to avoid a misleading low result.
If an initial result is below the reference range, the test should be repeated after at least 4 weeks. This gap helps distinguish a genuine inherited deficiency from a temporary acquired cause, such as an acute illness or recent clotting event.
A raised Free Protein S Antigen level is generally not associated with increased clotting or bleeding risk. High total Protein S antigen is considered to be of uncertain clinical significance because free Protein S levels are usually normal in such cases.
Protein S Antigen (Free) Citrated Plasma Test: Booking, Price, and Results
