Test Tube: Types, Sizes, Uses, and How It Differs From Other Lab Tubes

Look closely at a sugar fermentation broth and you may spot a tiny inverted tube sitting inside the larger test tube, and after incubation a small bubble of gas trapped at its closed top. That inverted tube is a Durham tube, and that trapped bubble is the whole point: it is how a microbiologist sees that an organism produced gas while fermenting the sugar, a result you cannot read any other way. It is a good reminder that in a microbiology lab a test tube is rarely just a container. Its size, its shape, and even a smaller tube tucked inside it are chosen to make a specific result visible.

Laboratory glassware is the most used laboratory equipment in various laboratories. Among many glasswares, test tube is applicable in biological as well as chemical laboratories.

The test tube is a long cylindrical handheld tube shaped like a finger that has been used in various experiments. It has different types depending on the material and purpose of the experiment.

Depending on the laboratory, test tubes also have different functions; in the chemistry laboratory, the test tube helps to test the properties of various chemicals and gasses. Likewise, the microbiology laboratory uses it to prepare culture media, collect samples, and hold fluids.

Test tubes are closed at the bottom and open at the top. The bottom of most test tubes is slightly curved/spherical. The length and diameter of the test tubes vary significantly from small (Durham) tubes to large culture tubes.

Figure: Test tube in rack

Sizes of Test Tubes

Test tubes are available in variable sizes; ranging from 10 to 20 mm wide and 50 to 200 mm in length. The standard test tube size used in laboratories is 18 mm✕150 mm.

1. Extra small (Durham) tube: about 5 mm × 35 mm or 4 mm × 40 mm. Placed upside down inside a larger tube of fermentation broth, it traps any gas produced by the organism, which appears as a visible bubble at its closed top. It is read as a positive test for gas production.
2. Small tube: about 13 mm × 100 mm, holding roughly 8–9 ml. Used for routine tests and biochemical reactions.
3. Medium tube: about 16 mm × 150 mm, holding roughly 15 ml. A common general-purpose size.
4. Large tube: about 20 mm × 150 mm, holding roughly 25–30 ml. Used for larger volumes and culture work.

Types of Test Tubes

This laboratory glassware are classified on different types on two basis; based on the material used for constructing them and based on the purpose.

Based on the material

Depending on the material used, test tubes are of two types; disposable and reusable.

1. Disposable test tubes: The disposable test tubes are made of various kinds of plastics like nylon, polyethylene, polypropylene, polyurethane, polyvinyl chloride, and polytetrafluoroethylene. Surprisingly, plastic test tubes are resistant to UV light like neoprene tubes, and different temperature ranges, like test tubes made up of nitrile operable in -30℉ to 275℉. The plastic tubes made up of polyethylene are resistant to high pressure.
2. Reusable test tubes: These are made of glass; borosilicate, fused silica, or quartz glass are some of the types of glass used for the test tubes. These tubes are highly resistant to temperature so autoclavable and tolerate chemicals. Although these tubes are transparent, they are more costly than disposable ones.

Other materials used in the construction of test tubes are metal and ceramic. However, using test tubes made of these materials is relatively uncommon.

Based on Purpose

Depending on the purpose, test tubes are of various types. Like thin-walled, medium-walled, thick-walled, rimmed, screw top, ground glass tops, plain tops, graduated test tubes, and some come with stoppers, push caps, and screwtops are some of the types of test tubes.

1. Thin-walled test tube: It is used for storage, holding, and mixing purposes but not heating due to the risk of easy breakage. The wall thickness in the tube is generally 0.5-0.6 mm.
2. Thick-walled test tube: It is used for strong heating of chemicals as the glass wall is thick. Its wall thickness is 1.5 mm.
3. Rimmed test tube: The rim of the test tube helps in providing extra mechanical strength in the test tube. It helps in easy holding using a test tube holder.
4. Screw top test tube: The screw top helps secure the materials inside the tube. It is usually helpful in sample collection, culture media preparation, and blood collection.
5. Plain top test tube: There is no rim or screw at the top of the test tube. It is useful for holding the chemicals before and during the experiment.
6. Medium-walled test tube: These tubes have a 1-1.2 mm wall thickness. It has usage in performing exothermic chemical reactions and gentle heating of materials.
7. Test tube with ground glass stopper: The tube is generally useful for storing chemicals and has a ground glass top to attach the stopper.
8. Graduated test tube: Like a graduated cylinder, it precisely measures samples before performing experiments. Usually, the tube has a thick wall and an interchangeable glass stopper.

Test Tube vs. Other Laboratory Tubes

Several tubes look similar but do different jobs, and confusing them is a common student mistake.

The distinctions that matter most: a centrifuge tube has a pointed bottom so the pellet collects at the tip, while a test tube is rounded; a culture tube is a test tube fitted with a cap for growing organisms without contamination; and a Durham tube is not a container at all but a gas trap placed inside another tube.

Uses of Test Tubes

Figure: Biochemical media in test tube

Test tubes have a wide range of functions depending on the laboratories. In chemical laboratories, these tubes test the substances’ properties and perform experiments in different forms between two or more chemicals. The various functions of test tubes are as follows:

1. Sample collection in clinical laboratories: The collection of samples like bodily fluids (saliva, pus, and sputum) occur in clean, sterilized test tubes. It helps to efficiently transfer samples from one place to another to confirm disease diagnosis.
2. Blood collection in phlebotomy: Blood collection tubes are specialized tubes with color-coded caps that indicate the additive inside and the tests they are used for. The color coding and order of draw are covered in detail in the blood collection tube article.
3. Culture media preparation in microbiological laboratories: The preparation of broth or liquid medium usually occurs in the test tube. Other biochemical mediums are also prepared in the test tube. It helps in the identification of microorganisms.
4. Analysis of chemicals: In the chemistry laboratory, a test tube is useful to study chemicals. The physical and chemical properties of chemicals in various physical states (solid, liquid, and gas) are analyzed by multiple tests using the test tube.
5. Disease diagnosis: In clinical and microbiology laboratories, test tubes hold the reactions and cultures used to diagnose disease, including biochemical identification tests and culture-based methods.
6. Holding or storing samples/chemicals: The use of some samples and chemicals may be frequent, so saving and keeping these chemicals and samples for the test and future tests is another usage of test tubes.
7. Heating experiments: Different experiment requires heating of the material for mixing or accurate result like Benedict’s tests, Fehling test, etc., and the material involved is small in amount. So, using a test tube helps in performing these experiments.

Why the Test Tube Shape Matters

The test tube's simple design is deliberate, and each feature serves a purpose worth understanding.

The narrow mouth reduces the exposed surface area, which limits contamination and evaporation and makes spills less likely than in a wide beaker. The rounded bottom lets contents mix and drain fully with no corners for material to lodge in, and it distributes heat evenly during gentle heating. The tall, narrow shape lets a small volume form a deep column, which is what makes a Durham tube's gas bubble or a settled cell layer easy to see. And the long body keeps the contents well below the mouth, so a tube can be held, heated, and inverted with less risk of the contents reaching your hand.

How to Remember

The bottom tells you the tube. The quickest way to identify a tube is to look at its bottom. Rounded means a general test tube or culture tube; pointed and tapered means a centrifuge tube, shaped so the pellet gathers at the tip. If you remember "round holds, cone separates," you will not mix them up.

The Durham tube catches a bubble. Its entire job is to trap gas so you can see it. A bubble at the top of the inverted tube after incubation means the organism produced gas. Picturing that single trapped bubble is enough to remember what the smallest tube in the lab is for.

Key Exam Facts in One Table

Where Students Get Confused

Test tube vs. culture tube. They look alike, but a culture tube usually has a cap and is used specifically to grow organisms without contamination. A plain test tube is the general-purpose version.

Test tube vs. centrifuge tube. The giveaway is the bottom: a centrifuge tube is tapered to a cone so the pellet collects at the tip and can withstand spinning, while a test tube is round-bottomed and not built for high-speed centrifugation.

What the Durham tube actually does. Students often think the small inverted tube holds a sample. It holds nothing at the start; it is there to trap gas produced during fermentation, and the trapped bubble is the result being read.

Glass vs. plastic choice. Reusable borosilicate glass is autoclavable and handles heat and chemicals; disposable plastic tubes are cheaper and avoid cross-contamination but cannot be strongly heated. The choice depends on whether the tube will be heated, autoclaved, or reused.

References

• Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
• Cheesbrough, M. (2006). District Laboratory Practice in Tropical Countries, Part 2 (2nd ed.). Cambridge University Press.
• Bayot, M. L., & Tadi, P. (2023). Laboratory Tube Collection. In StatPearls. StatPearls Publishing. PMID: 32310451.
• Dimeski, G., Yow, K. S., & Brown, N. N. (2015). What is the most suitable blood collection tube for glucose estimation? Annals of Clinical Biochemistry, 52(2), 270–275. https://doi.org/10.1177/0004563214544708