How many mL is a tube of blood?

Quick Answer

The amount of blood drawn into a tube depends on the size of the tube. Common tube sizes are:

  • 4-5 mL for a red top tube
  • 2-3 mL for a purple top EDTA tube
  • 1-3 mL for a blue top citrate tube

So in summary, a standard tube holds between 1-5 mL of blood depending on the type and size of the tube.

How Much Blood is Drawn for Common Blood Tests?

The amount of blood needed for different types of blood tests can vary. Here are some general guidelines:

Complete Blood Count (CBC)

For a complete blood count, usually one tube with 2-3 mL of blood is needed. This is one of the most common blood tests.

Basic Metabolic Panel

For a basic metabolic panel, one tube with 4-5 mL of blood is typically required. This checks electrolytes, kidney function, blood sugar, and more.

Lipid Panel

For a lipid panel to check cholesterol levels, one 4-5 mL tube is usually sufficient.

Blood Cultures

For blood cultures to check for infections, each culture bottle usually requires 5-10 mL of blood. Often two sets are drawn from different sites.

So the amount can range quite a bit depending on what specifically is being tested. But most standard blood tests require between 2-5 mL collected into vacuum tubes.

What Are the Different Tube Types and Additives?

There are a few main types of blood collection tubes, each with different additives and purposes:

Red Top Tube

  • Tube color: Red (glass) or gold (plastic)
  • Additive: None
  • Use: Chemistry tests, blood counts
  • Volume: 4-5 mL

Purple Top EDTA Tube

  • Tube color: Lavender/purple
  • Additive: EDTA anticoagulant
  • Use: Hematology testing, blood counts
  • Volume: 2-3 mL

Blue Top Citrate Tube

  • Tube color: Light blue
  • Additive: Sodium citrate anticoagulant
  • Use: Coagulation studies
  • Volume: 1-3 mL

There are some other less common tube types too like green top (heparin) and gray top (fluoride/oxalate), but the red, purple, and blue top tubes are the most widely used. The additive in each helps preserve the blood sample for different types of diagnostic testing.

What Are the Steps for Drawing Blood Into Tubes?

When drawing blood into vacuum tubes, there are some specific steps medical personnel follow:

  1. Assemble supplies: tubes, needle, tourniquet, alcohol wipes, gauze, sharps container
  2. Identify the patient and explain the procedure
  3. Apply the tourniquet 3-4 inches above the venipuncture site
  4. Choose the vein and clean the site with an alcohol wipe
  5. Insert the needle into the vein and secure it with tape
  6. Place the first tube into the holder and push onto the needle to draw blood
  7. When the tube is full, remove it and continue filling additional tubes
  8. Remove the tourniquet once all tubes are filled
  9. Withdraw the needle and immediately apply gauze to the site
  10. Dispose of the used needle safely into a sharps container
  11. Invert the tubes gently to mix the additives
  12. Label the tubes clearly and send to the lab

The phlebotomist draws enough blood to fill each tube completely. The vacuum in the tube draws the blood in once it’s inserted into the holder. The tubes must be handled properly to ensure accurate test results.

What Are Reasons for Different Tube Volumes?

There are a few main reasons why blood collection tubes come in different volumes:

Amount of Blood Needed

Some tests require larger blood samples than others. For example, a CBC may need only 2-3 mL, while a potassium level needs a full tube of 4-5 mL. The tube size correlates to the minimum amount of blood required.

Additive-to-Blood Ratio

The proper ratio of additive to blood must be maintained for accurate results. A smaller lavender top tube has less EDTA for the volume of blood than the larger size. The size is chosen to give the ideal ratio for that test.

Pediatric Testing

Smaller volume tubes are available for pediatric patients. Children only require 1 mL or less of blood, so special micro-tubes have been designed to reduce sample size.

Multiple Tests

Drawing larger tubes allows blood to be divided for several tests. One red top tube can provide enough blood for a CBC plus a basic metabolic panel if split correctly.

Tube Holder Size

Standard needle holders can only accommodate tubes up to a certain size before they become too wide. So large diameter tubes aren’t practical for routine collection.

Typical Tube Volumes Used for Common Blood Tests

Blood Test Tube Type Volume Needed
Complete blood count Purple top EDTA 2-3 mL
Comprehensive metabolic panel Red top 4-5 mL
Lipid profile Red top 4-5 mL
PTT/PT/INR (coagulation) Blue top citrate 1-2 mL
Blood cultures Blood culture bottles 5-10 mL per bottle x 2

This table summarizes the most common tube additives used and the typical volumes drawn for routine blood tests. This provides a reference for how much blood is generally needed for each type of analysis.

Special Considerations for Blood Volume

While standard tubes hold set amounts, there are some situations where blood volume needs special consideration:

Pediatric Patients

Smaller tubes and needles are used to reduce the volume needed from children. Still, proper amounts must be drawn for accurate results.

Obese Patients

Those with high body fat may need extension sets for blood tubes to reach veins. But the tube size itself does not usually need adjustment.

Severely Anemic Patients

The volume drawn may be minimized if the patient has very low hematocrit or hemoglobin levels. However, a certain minimum volume is still needed for each test.

Difficult Venipuncture

If drawing blood is challenging, smaller tubes may be used and prioritized for key tests to conserve the limited amount obtained.

Research Studies

Some research studies only need small samples. Capillary tubes of <1 mL may be used if venous access is not required. So while standard tubes are used for most patients, the blood volume needs to be optimized in certain situations. The medical team determines the best approach based on the clinical scenario.

Tube Size and Volume Variability

While tubes are engineered to standard volumes, there is some slight variability:

– Plastic tubes generally hold slightly less than glass tubes.

– There is up to 10% variability between different manufacturers.

– The vacuum pressure can affect the final volume up to 5%.

– Short draws or under-filled tubes may provide less than the stated volume.

So the expected fill volumes listed on tubes are typical but not absolute. The actual volume can vary a bit based on these factors.

Key Takeaways

  • Standard blood collection tubes range from 1-5 mL in volume.
  • Purple top EDTA tubes for CBCs require 2-3 mL.
  • Red top and blue top tubes for chemistry and coagulation tests use 4-5 mL.
  • Volume needed depends on test, additive, and minimum blood required.
  • Tube size is optimized to give the ideal ratio of blood to additive.
  • Smaller volumes may be needed for pediatric patients.
  • There is some natural variability in volumes between manufacturers.

So in summary, the amount of blood in a tube depends on the type and size, but is generally between 1-5 mL for routine testing. This provides an optimal amount for sample analysis while minimizing the blood volume required from each patient.

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