I have been asked by several patients on numerous occasions, “how much blood are you going to take?” Or “how many vials do you have to take?”
Depending on the number of tests the patient is doing, I will usually respond and tell them, just one vial or half a vial or two or three.
On one occasion, after taking just half a vial for this one man, he said to me, “You take all my blood, man?” I replied, “No man! Just a little bit I took!” 😀
So, how much blood is drawn for a blood test? The amount of blood drawn for a PT, PTT or D-dimer is between 2.2-4.0 ml depending on the size of the tube. For most other tests, the amount drawn is anywhere from 1 to 25 ml or 1 to 5 vials depending on the number of tests that have to be done.
There are several factors that determine how much blood is drawn for a blood test. Let’s explore them below.
The Type Of Test
Some tests have specific requirements. A PT or PTT test is one of them. A light blue or a dark blue tube, depending on the brand is used. The tube contains sodium citrate, a clear liquid solution, at the bottom.
Depending on the brand of tube, the volume may be different. I have seen 2.7 ml and 3.5 ml. So, the 2.7 ml contains less sodium citrate than the 3.5 ml.
The amount of blood drawn must be proportionate to the amount of sodium citrate inside the tube. To ensure the correct amount of blood is drawn into the tube, there is a marking near the top indicating how much is needed.
The tube must be filled to this marking. Under-filling or over-filling it could result in erroneous results.
Table Showing Three tube sizes for a PT or PTT test.
| Brand | Color | Maximum Volume | Amount Needed for Test |
| Covidien | Light Blue | 4.5 ml | About 4 ml |
| Vacuette | Dark Blue | 3.5 ml | About 3 ml |
| BD vacutainer | Light Blue | 2.7 ml | About 2.2 ml |
Erythrocyte Sedimentation Rate
Erythrocyte sedimentation rate or ESR is another test that needs a specific amount of blood. To do an ESR, 1 ml of whole blood is needed.
But the person collecting the blood would not draw exactly 1 ml. About 2 ml would be preferable.
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Number Of Tests Being Done By Patient
The number of tests being done will vary from person to person. One person may just do a CBC or PT whilst another person may have to do a general health panel, which includes a Comprehensive Metabolic Panel, Lipid Profile, CBC, and Urinalysis.
The amount of blood drawn will depend on the number of tests requested. Using real life examples, let’s assume 3 persons have to do different blood tests.
John’s Doctor has sent him to do a CBC.
Mark wants to do a routine check up. His Doctor has sent him to do a General Health Profile.
Craig wants to do an Infectious Profile. His Doctor has sent him to do HIV, Syphilis, Gonorrhea, Glamydia, and HSV.
Let’s take each request above and find out how much blood that would be needed for each person.
| Patient | Test | Tube | Volume Needed |
| John | CBC | Lavender | 1-2 ml |
| Mark | General Health | Lavender Red or SST | 1-2 ml About 3-6 ml |
| Craig | Infectious Profile | Red/tiger/brown | About 2-4 ml |
Collection Tube Sizes
There are many tubes that are being used to collect blood.
- Adult tubes generally hold from 2 to 10 ml of blood
- Children tubes usually hold from 2 to 4 ml of blood
- Tubes for fingersticks or heelsticks generally hold 1 half ml or less
Below is a table showing some of the most basic tubes used and the volume of each.
| Tube | Brand | Tube Volume |
| Light/dark blue (Sodium Citrate) | BD Vacutainer Vacuette Covidien | 2.7 ml 3.5 ml 4.5 ml |
| Lavender (EDTA) | Vacuette BD Vacutainer | 4 ml 4 ml |
| Gray (Sodium Fluoride/Potassium Oxalate) | Vacuette BD Vacutainer | 4 ml 3-10 ml |
| Red/brown/tiger SST | Vacuette BD Vacutainer | 2-10 ml 4-10 ml |
| Green (Sodium Heparin/Lithium Heparin) | Vacuette BD Vacutainer | 4-10 ml 4-10 ml |
Many tests can be done on the same sample of blood that was drawn. For example, a Serum Separator Tube or SST is used for most chemistries and one tube is sufficient to do all chemistry tests.
So, taking a single sample for each test is not necessary. Another example is a CBC and an ESR. Both can be done using the same collection tube.
Differences From One Facility To Another
Be aware that the amount of blood drawn may vary from facility to facility and laboratory to laboratory.
Each facility or laboratory has their own policy and requirements for the amount of blood that would be drawn from a person doing a blood test. The type of tubes used may also vary.
Also, the amount of blood drawn for a specific test will not be exactly equal to the amount needed for that test. There are several reasons for drawing more blood than is needed. Here are a few:
- Repeating the test.
- Further testing.
- Sending sample to a reference lab.
- In case of a compromise or an accident.
- Storing away the sample. More tests can be requested for the patient.
Think of a teaspoon and a tablespoon. A teaspoon is equivalent to about 5 ml. A tablespoon is equivalent to about 10 ml.
When you do a CBC about 1-2 ml of blood is drawn. When you do a PT/INR test about 2.2-4 ml of blood is drawn. The amount drawn is about a quarter to 3 quarters of a teaspoon.
If you want a clearer picture of how much of your blood is drawn when you do a blood test, think of a teaspoon or tablespoon 😀
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Conclusion
So, how much blood is drawn for a blood test? About 1 to 25 ml or 1 to 5 vials of blood is drawn depending on the type of test and the number of tests the person is doing.