Choosing the Right Test Tube

Test tubes are used for many laboratory applications. Here are the main ones:

• Biology: In microbiology, test tubes are often used to grow microorganisms.
• Clinical applications: Test tubes are used to prepare and test biological samples, such as blood or urine. There are several types of blood collection tubes, which are distinguished by the color of their caps (see question 2). They are commonly called VacutainerⓇ tubes, after the brand that originated this type of test tube.
• Centrifugation: Certain test tubes are used specifically for centrifuges.
• Chemistry: Test tubes are often used to test chemical reactions on small volumes of reactants.
• Sample storage: Test tubes are used for stocking samples in biobanks.
• General laboratory applications: Test tubes are used in a wide range of laboratory applications, for example, to buffer solutions, handle liquid reactants, etc.

Depending on the analyses carried out on the blood sample, there are different color codes to distinguish the test tubes and avoid analysis errors. There are seven main colors: red, yellow, blue, purple, green, grey, and light mauve.

• Red test tubes: Test tubes with a red cap do not contain any anticoagulants but instead contain a coagulation activator (silicon microparticles). After centrifugation, a serum rather than plasma is obtained, as with tubes containing an anticoagulant. This tube is used in neurology, biochemistry (ionograms, urea tests, creatinine tests, cholesterol tests), immunology, hormonology, allergology, and oncology.
• Yellow test tubes: Test tubes with a yellow cap are used for the same analyses as red test tubes and have the same properties, except they also contain a gel. After centrifugation, this gel acts as a separator between the liquid part of the sample and the cells, preventing them from mixing again.
• Blue test tubes: Test tubes with a blue cap contain an anticoagulant, sodium citrate. It is used for coagulation tests and to monitor anticoagulant treatments.
• Purple test tubes: Test tubes with a purple cap also contain an anticoagulant, EDTA. It is useful for measuring the number of white blood cells, red blood cells, and platelets, or even for determining blood groups and analyzing glycated hemoglobin.
• Green test tubes: Test tubes with a green cap contain an anticoagulant, lithium heparin, which is used for a few very specific analyses, such as lactate or methemoglobin.
• Grey test tubes: Test tubes with grey caps contain an anticoagulant, sodium fluoride, mainly used for blood glucose testing, as fluoride prevents the breakdown of glucose by blood cells in the event of prolonged contact.
• Light mauve test tubes: These are intended for pediatric use and have a smaller capacity.

There are two materials used to make test tubes: glass and plastic. Here are their respective characteristics:

• Glass test tubes: Glass test tubes are generally made of borosilicate glass. They are used for high-temperature applications, such as chemical experiments.
• Plastic test tubes: Test tubes can also be made of different plastic materials. There are several types available:
  • Polypropylene (PP)
  • Polycarbonate
  • Polyethylene (PET)
  • Polystyrene
  • PMMA

Test tubes are always cylindrical but the shape of their base can vary. There are three different shapes: flat, round, or conical.

• Flat-bottom test tubes are used for clinical, environmental, industrial (e.g. dairy industry), or microbiological applications.
• Round bottom test tubes: These are ideal for laboratory tests and clinical, food-processing, and industrial applications.
• Cone-bottom test tubes: These are mainly used for refrigerated and non-refrigerated centrifuges. They offer high mechanical strength and good clarity thanks to their thin, smooth walls.

Flat-bottom test tubes

Ratiolab cone-bottom test tube

Simport round-bottom test tube

Depending on the application, certain test tube characteristics may be important to consider, such as graduation, sterility, or even whether or not it has a rim.

• Graduation: Many test tube models are graduated, which can be useful in applications where it is necessary to measure the quantities of quantifiers or manipulated reactors.
• Sterility: This feature guarantees that the samples being handled will not be contaminated by the test tube.
• Single-use or reusable test tubes: Depending on the application, it may be better to use single-use test tubes, especially when preparing physiological samples such as blood.
• The type of cap: Screw caps are recommended. These caps can be attached or unattached.
• With a rim or rimless: A test tube with a rim allows for easier pouring.
• Capacity: Depending on the applications and the volume of the samples and reactors, you can choose between a large or small capacity. Pediatric test tubes with a light mauve cap have a smaller capacity.
• Dimensions: The dimensions of the test tube must considered, especially when they need to be placed on racks. It is therefore essential that the dimensions of the slots or holes in the rack are compatible with the size of the test tubes.