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A capillary tube is a thin tube, usually glass, that draws liquid in by capillary action. This page covers the laboratory and clinical meaning: the disposable glass blood sample capillary tube used in microhematocrit testing. (The same words also name a refrigerant metering tube in HVAC equipment, which is a different product and is not covered here.)
This page describes products and general use; it is not medical advice.
What is a capillary tube?
A blood sample capillary tube is a small disposable glass tube that fills with a drop of blood by capillary action. ASTM E734 covers disposable glass blood sample capillary tubes made for use in microhematocrit procedures. The microhematocrit method centrifuges the filled tube so the red cells pack at one end, which allows the packed cell volume (PCV) to be measured directly.
Who uses it: laboratory and clinic staff who run microhematocrit (packed cell volume) tests,.
How is a capillary tube built?
ASTM E734-80(2021) sets the material and construction requirements. Per the standard’s abstract:
- The tubes are made from Type I, Class B borosilicate glass, or Type II soda lime glass.
- Where a tube is coated with heparin, the heparin is the ammonium salt, isolated from beef or pork lung or intestinal mucosa.
- The standard sets requirements for design, dimension, workmanship, color coding and lot or control number.
- Tubes are tested for capillarity, fluidity, sheep plasma, positive and negative controls, human whole blood, heparin potency and resistance to centrifugal force.
Types of capillary tube
ASTM E734 recognises two types, distinguished by whether the inside is coated.
| Type | Coating | Source |
|---|---|---|
| Type I | Coated with heparin | ASTM E734-80(2021) |
| Type II | Uncoated | ASTM E734-80(2021) |
The glass type (borosilicate or soda lime) is a separate property listed in the same standard; the abstract does not tie one glass to one type.
Where is it used? The microhematocrit method
CLSI H07-A3, Procedure for Determining Packed Cell Volume by the Microhematocrit Method, describes a standard method for direct measurement of packed cell volume by centrifugation. CLSI lists it as needed for whole-blood calibration of instrumental methods. CLSI shows the document as archived (no longer under consensus review) while still technically valid, and FDA recognizes it for regulatory use. The capillary tube is the container in which that centrifugation takes place.
Capillary blood samples are often taken with a lancet; see what a lancet is. Analyzers that work from small blood samples are covered in what a blood gas analyzer is, and cell counting by a different method is covered in the hemocytometer cell counting guide. For handling used sharps, see sharps disposal regulations for laboratories. For protective clothing worn when handling specimens, see medical apparel.
Frequently asked questions
What is a capillary tube used for in a lab?
Blood sample capillary tubes are used in microhematocrit procedures, where the filled tube is centrifuged to measure packed cell volume.
What is the difference between heparinized and plain capillary tubes?
In ASTM E734, Type I tubes are coated with heparin and Type II tubes are uncoated.
What are capillary tubes made of?
ASTM E734 specifies Type I, Class B borosilicate glass or Type II soda lime glass.
Is the capillary tube in a refrigerator the same thing?
No. HVAC systems use a long, narrow tube as a refrigerant metering device. This page covers only the laboratory blood sample tube.
Sources
- ASTM E734-80(2021), Standard Specification for Disposable Glass Blood Sample Capillary Tube (Microhematocrit), https://store.astm.org/e0734-80r21.html, abstract checked 2026-10-29.
- CLSI H07-A3, Procedure for Determining Packed Cell Volume by the Microhematocrit Method, https://clsi.org/shop/standards/h07/, checked 2026-10-29.
