How to Choose Serum Blood Collection Tubes for U.S. Laboratories

Summary

• For routine serum testing in the United States, purchase plastic gold-top serum separator tubes (SSTs) or red-and-gray tiger-top SSTs when the laboratory’s test menu and analyzer instructions validate their use.
• Use plain red-top tubes when the laboratory requires serum without Separator Gel or Clot Activator, and use specialty tubes only when a specific assay, specimen type, or reference laboratory requires them.
• Choose tubes by validated test compatibility, fill volume, specimen stability, safety requirements, regulatory status, and total cost rather than cap color alone.

Purchasing the correct blood collection tubes for serum samples is essential for accurate laboratory results, efficient phlebotomy workflows, and compliance with United States laboratory requirements. Although tube colors provide a useful visual shortcut, color alone should never be the sole basis for procurement. Different manufacturers may use similar colors for tubes with different additives, separator materials, draw volumes, or intended applications.

For most routine serum testing in the United States, buyers generally evaluate three main options: plain red-top tubes, gold-top serum separator tubes, and red-and-gray or “tiger-top” serum separator tubes. The best choice depends on the tests being performed, the laboratory’s validated procedures, the instruments in use, specimen transport conditions, and whether the sample will be sent to an external reference laboratory.

Clinical laboratories process enormous numbers of blood specimens every year. Data from the Centers for Disease Control and Prevention, the Centers for Medicare & Medicaid Services, and national laboratory industry reports demonstrate the scale of testing in the United States. The country has more than 260,000 federally certified clinical laboratories under the Clinical Laboratory Improvement Amendments program, although this total includes facilities with different testing volumes and complexity levels. Blood collection is among the most frequently performed healthcare procedures, and even a small rate of preanalytical error can affect thousands of specimens across a large health system.

Reports from professional laboratory organizations commonly identify preanalytical problems—including incorrect tube selection, insufficient volume, hemolysis, clotting, mislabeling, and delayed processing—as a major share of laboratory errors. Selecting a serum tube that is appropriate for the test menu is therefore both a purchasing decision and a patient-safety decision.

Understanding Serum and Serum Collection Tubes

Serum is the liquid portion of blood remaining after the blood has clotted and the clot has been removed. Unlike plasma, serum does not contain fibrinogen and several other clotting proteins consumed during coagulation. Serum is widely used for chemistry, immunology, serology, endocrinology, Therapeutic Drug Monitoring, and many infectious disease assays.

A serum tube usually contains one of the following configurations:

1. A plain tube with no additive, allowing the blood to clot naturally.
2. A tube containing a Clot Activator to accelerate coagulation.
3. A tube containing a Clot Activator and a polymer gel that separates serum from the clot during centrifugation.
4. A specialty tube designed for a particular test, transport condition, or laboratory platform.

The tube must be allowed to clot before centrifugation. If the sample is centrifuged too early, fibrin strands may remain in the serum and interfere with analyzers, pipetting systems, or immunoassay platforms. If it sits too long before processing, analyte concentrations may change and the risk of hemolysis or specimen degradation may increase.

Most serum tubes are made from plastic, commonly polyethylene terephthalate, rather than glass. Plastic tubes are lighter and less likely to break, supporting workplace safety and modern specimen transport systems. However, plastic tubes may require clot activators because blood may clot more slowly on plastic than on glass.

The Main Serum Tube Options

Gold-Top Serum Separator Tubes

Gold-top serum separator tubes, commonly called SSTs, are the standard purchase for many routine serum chemistry and immunology applications. They typically contain a Clot Activator and a polymer gel. During centrifugation, the gel moves between the clot and serum, forming a physical barrier that helps protect the serum from cellular contamination.

Gold-top SSTs are commonly used for comprehensive metabolic panels, basic metabolic panels, lipid profiles, liver function testing, thyroid testing, hormone assays, many antibody tests, and numerous routine chemistry procedures. They are especially useful in high-volume outpatient and hospital laboratories because they can reduce the need for manual serum transfer.

The gold cap is widely recognized in the United States, but procurement teams must confirm the manufacturer’s product specification. Some gold tubes contain a Separator Gel and Clot Activator, while other tubes with similar colors may have different formulations. The laboratory should verify the exact product number, tube dimensions, nominal draw volume, additive, gel characteristics, and validated test list.

Common draw sizes include approximately 3.5 milliliters, 5 milliliters, 7 milliliters, and 8.5 milliliters, although availability varies by manufacturer. Smaller tubes may be appropriate for pediatric or Geriatric Patients, while larger tubes may be necessary for multiple tests or send-out testing.

Red-and-Gray Tiger-Top Serum Separator Tubes

Red-and-gray tiger-top tubes are another common form of SST. The mottled or marbled appearance is intended to distinguish them from solid-color tubes. They generally contain a Clot Activator and Separator Gel and are used for many of the same routine serum applications as gold-top tubes.

Tiger-top tubes can be practical when a facility already uses them across inpatient, outpatient, and reference-laboratory workflows. They are not automatically interchangeable with every gold-top tube, however. The tube’s internal gel, Clot Activator, fill volume, and validated performance must be reviewed before substitution.

When purchasing tiger-top or gold-top tubes, request documentation showing the intended specimen type and applicable testing categories. A tube that performs well for routine chemistry may not be suitable for every Toxicology, therapeutic drug, trace-element, molecular, or specialized Immunology Test.

Plain Red-Top Tubes

Plain red-top tubes contain no anticoagulant. Depending on the product, they may be glass or plastic. A plain tube allows blood to clot without the presence of gel or, in some products, without a Clot Activator. After clot formation and centrifugation, the serum is separated from the clot.

Red-top tubes are useful when a laboratory does not want Separator Gel or when a reference laboratory specifically requests a non-gel serum specimen. They may be preferred for certain therapeutic drug assays, specialized chemistry methods, blood bank-related procedures, or tests in which gel contact has been shown to cause interference.

Because plain red-top tubes may clot more slowly than tubes containing Clot Activator, staff must follow the laboratory’s required Clotting Time. A common practice is to allow the specimen to clot for approximately 30 minutes before centrifugation, but the exact interval should be based on the tube manufacturer’s instructions and the laboratory’s validated procedure.

Red-top tubes are also useful as a backup product when a test directory specifies “serum, red-top” rather than “serum separator.” They should not be purchased as a universal replacement for SSTs. Using a plain tube for a test validated only with an SST may create processing delays or require manual transfer.

Specialty Serum Tubes

Some serum tests require specialty tubes with a particular additive, surface treatment, low-binding material, or trace-element specification. Examples may include tubes used for metals testing, Therapeutic Drug Monitoring, hormone assays, allergen testing, or samples that must be protected from contamination.

Trace-element testing deserves particular attention. Tubes intended for lead, mercury, arsenic, cadmium, and other metals may require certified low-background or metal-free materials. A routine red-top or gold-top tube may introduce contamination and produce misleading results. Procurement teams should purchase these tubes only according to the laboratory’s test-specific instructions and acceptance criteria.

Some tests require glass tubes, while others require plastic. Some assays require serum to be transferred into a secondary transport vial after centrifugation. Certain reference laboratories may reject specimens collected in gel tubes because of drug adsorption, gel carryover, or instrument-specific concerns. A test directory should control these decisions.

How Tube Color Should Influence Purchasing

Tube color is a useful identification system, but it is not a national product standard that guarantees identical contents across all brands. In the United States, tube colors are strongly associated with common additives, but manufacturers may use variations in cap color, label design, tube size, and additive formulation.

Before approving a product, compare the following characteristics:

1. Cap color and visual identification.
2. Additive type, including Clot Activator and Separator Gel.
3. Nominal blood draw volume and acceptable fill range.
4. Tube material, such as glass or plastic.
5. Tube dimensions and compatibility with centrifuges and automated lines.
6. Expiration date and shelf-life expectations.
7. Storage requirements and temperature limits.
8. Manufacturer instructions for clotting, centrifugation, and specimen handling.
9. Regulatory status and documentation for in vitro diagnostic use.
10. Compatibility with the laboratory’s analyzers and reference laboratories.

The correct approach is to define the required specimen by additive and validated application, then use color as a secondary identification aid.

Matching Tubes to the Laboratory Test Menu

The most reliable purchasing method begins with a review of the laboratory’s test menu. Divide tests into categories and identify which serum tube each assay requires. Routine chemistry may use an SST, but specialized procedures may require a plain red-top or a dedicated specialty tube.

A practical test-to-tube matrix should identify the preferred tube, acceptable alternative, minimum volume, processing time, storage temperature, and rejection criteria. The matrix should be approved by the laboratory director or another authorized technical supervisor.

For example, a routine outpatient laboratory may find that gold-top SSTs meet the requirements for most chemistry and immunology testing. A hospital laboratory with Therapeutic Drug Monitoring, Toxicology, endocrinology, and send-out testing may need both SSTs and plain red-top tubes. A pediatric facility may need short-draw and low-volume serum tubes in addition to standard tubes.

Testing volume should also influence the product mix. If 80% of serum specimens are routine chemistry and 20% require non-gel specimens, purchasing only one tube type will create unnecessary substitutions. Conversely, stocking too many specialty tubes increases inventory expense, expiration risk, and the chance of selecting the wrong product.

Important Specimen Collection and Processing Requirements

Order of Draw

The Clinical and Laboratory Standards Institute’s venous blood collection guidance, commonly identified as GP41, provides the basis for modern order-of-draw practices. Serum tubes are generally collected after Blood Culture bottles and tubes used for coagulation testing, and before many tubes containing anticoagulants such as EDTA or Sodium Fluoride. The exact sequence depends on the collection system and institutional policy.

Following the order of draw helps reduce additive carryover. For example, contamination from EDTA can produce falsely increased potassium and falsely decreased calcium or magnesium results. Cross-Contamination from anticoagulants may create Specimen Rejection, repeat collection, delayed diagnosis, or incorrect treatment.

Mixing the Tube

Clot Activator tubes should generally be gently inverted according to the manufacturer’s instructions. Vigorous shaking can cause hemolysis, while inadequate mixing may produce microclots or incomplete activation. Plain tubes may require no additive mixing, but staff should still handle them carefully and follow the product instructions.

The exact number of inversions varies by manufacturer. Procurement and nursing education materials should use the instructions supplied with the selected tube rather than relying on a generic number copied from another product.

Clotting Time

Serum tubes must remain undisturbed until a complete clot forms. The minimum Clotting Time depends on whether the tube contains a Clot Activator, the patient’s coagulation status, and the laboratory’s procedure. Patients taking anticoagulants or those with Clotting Disorders may require additional time.

Failure to allow complete clotting can produce fibrin strands. These strands may clog sample probes, interfere with automated aliquoting, and cause inconsistent results. A laboratory should define when a specimen must be recollected or manually processed.

Centrifugation

Centrifugation must comply with the tube manufacturer’s recommended relative centrifugal force, time, and temperature. Revolutions per minute alone are not enough because the effective force varies according to rotor radius. Laboratories should document relative centrifugal force in their procedures.

Gel tubes are designed to create a barrier after centrifugation. If the gel does not form a stable barrier, the tube may not be compatible with the centrifuge program, may be underfilled or overfilled, or may have been stored improperly. A failed barrier can allow cellular contact with serum and create problems in transport or analysis.

Fill Volume and Low-Volume Tubes

Correct fill volume matters even for serum tubes. Underfilling may reduce the amount of serum available for testing, especially when multiple assays are ordered. It may also affect the ratio of blood to additive in tubes containing clot activators or gel systems.

Low-volume tubes can reduce blood loss in pediatric, neonatal, oncology, intensive-care, and geriatric populations. However, smaller tubes are not automatically appropriate for every analyzer. The laboratory must verify that the resulting serum volume is sufficient for the primary test, repeat testing, dilution, Quality Control, and possible send-out requirements.

When evaluating pediatric tubes, consider dead volume in automated analyzers. A tube that contains 0.8 milliliters of blood may yield substantially less usable serum after clotting, gel retention, and instrument aspiration. Product specifications should state the approximate serum yield, not simply the nominal draw volume.

Short-draw tubes can also create operational complexity if staff confuse them with standard tubes. Distinct labels, barcode rules, storage locations, and staff training help prevent errors.

Regulatory and Quality Considerations in the United States

Clinical laboratories performing testing on human specimens are generally subject to CLIA requirements. The laboratory director is responsible for ensuring that testing procedures are appropriate, validated, and monitored. A change in blood collection tube can be a method or specimen-system change that requires verification before routine use.

The U.S. Food and Drug Administration regulates many blood collection tubes as medical devices or in vitro diagnostic products. Buyers should verify that the product is marketed for the intended use and that the supplier provides appropriate labeling, instructions, lot information, and quality documentation.

Accrediting organizations such as the College of American Pathologists and The Joint Commission may evaluate specimen collection, identification, handling, and quality systems. A tube purchasing decision should therefore be supported by a documented evaluation rather than an informal substitution based only on price.

Before introducing a new tube, the laboratory may need to compare patient specimens collected in the current and proposed products. Evaluation criteria can include test bias, precision, hemolysis rates, clotting performance, gel barrier formation, specimen stability, analyzer compatibility, and rejection rates. The extent of verification should reflect the risk and the laboratory’s policy.

Analyzing Cost Beyond the Unit Price

The lowest price per tube is not necessarily the lowest total cost. A less expensive product may have a higher rate of leakage, breakage, hemolysis, insufficient volume, or analyzer problems. Each rejected specimen can generate labor costs, recollection costs, delayed treatment, patient dissatisfaction, and potentially additional transportation expense.

Total cost of ownership should include:

1. Purchase price per tube.
2. Freight, hazardous-material handling, and delivery charges.
3. Storage space and inventory management.
4. Expiration-related waste.
5. Labels, racks, adapters, and compatible accessories.
6. Labor required for manual transfers or special processing.
7. Specimen Rejection and recollection costs.
8. Equipment changes or analyzer-specific consumables.
9. Quality-control and validation costs.
10. Emergency or expedited shipping when stock is unavailable.

Large health systems may negotiate tiered pricing based on annual volume, but volume commitments should be balanced against expiration dates and storage capacity. A discount is not beneficial if the facility must discard thousands of expired tubes.

Supplier Evaluation and Inventory Planning

Choose suppliers that can provide consistent lot availability, reliable delivery, technical support, and transparent product documentation. Ask whether the supplier can maintain the same product specification across lots and whether advance notice is provided for changes in raw materials, gel formulation, cap design, or manufacturing location.

At least two qualified supply sources may be desirable for critical products, but automatic substitution should be prohibited unless the substitute has been approved. A visually similar tube from another manufacturer may contain a different gel or Clot Activator.

Use historical consumption data to forecast demand. Calculate average monthly use by department, then add a safety stock based on lead time, supply-chain volatility, seasonal demand, and emergency requirements. Inventory software should track lot numbers and expiration dates using a first-expiring, first-out method.

During shortages, do not substitute a different serum tube solely because the cap color looks similar. Contact the laboratory director, review the test directory, and consult the manufacturer’s technical documentation. Some tests can tolerate an alternative tube, while others cannot.

Common Purchasing Mistakes to Avoid

1. Buying tubes based only on cap color without confirming the additive.
2. Assuming every gold-top tube is interchangeable with every other gold-top tube.
3. Using gel tubes for tests that require plain serum or a non-gel specimen.
4. Ignoring minimum specimen volume and analyzer dead volume.
5. Failing to verify centrifuge compatibility.
6. Mixing products from different manufacturers without validation.
7. Overstocking specialty tubes with low demand.
8. Failing to monitor expiration dates and lot numbers.
9. Using tubes that are not approved for the intended diagnostic purpose.
10. Changing tubes without updating collection manuals, training, and electronic test directories.

Another frequent mistake is purchasing serum tubes for tests that actually require plasma, whole blood, or a specialized transport medium. The specimen type should be confirmed at the assay level. A patient’s diagnosis or department name is not sufficient to determine the correct tube.

A Practical Purchasing Recommendation

For a typical U.S. outpatient or hospital laboratory performing routine serum chemistry and immunology, the primary purchase is usually a plastic gold-top SST or a red-and-gray tiger-top SST with Clot Activator and Separator Gel. Select the tube size based on patient population, average test volume, and analyzer requirements.

Maintain a secondary inventory of plain red-top tubes for tests that prohibit gel or require non-gel serum. Add specialty serum tubes only when the test directory, reference laboratory, or assay manufacturer specifically requires them. Pediatric and low-volume facilities should also evaluate short-draw serum tubes, but only after verifying specimen yield and instrument compatibility.

Before issuing a purchase order, obtain product samples and complete a documented review. Confirm the additive, tube material, draw volume, clotting instructions, centrifugation requirements, storage conditions, expiration period, and regulatory labeling. Then ask the laboratory director or qualified designee to approve the product for the intended tests.

Final Decision Checklist

A purchasing team can use the following sequence to make a defensible decision:

1. List every serum-based test performed or referred by the facility.
2. Identify the required tube for each test from the laboratory procedure or reference-laboratory instructions.
3. Separate routine SST-compatible tests from plain-serum and specialty-tube tests.
4. Estimate monthly usage by tube size and department.
5. Compare products by additive, material, volume, quality documentation, and total cost.
6. Verify compatibility with centrifuges, analyzers, Pneumatic Tube Systems, and specimen transport.
7. Conduct laboratory verification before implementation.
8. Update standard operating procedures, electronic ordering systems, and staff training.
9. Monitor hemolysis, rejection, recollection, leakage, and expiration waste after launch.
10. Review the product periodically and whenever the manufacturer, assay, or analyzer changes.

In summary, most U.S. facilities should begin with a validated gold-top or tiger-top serum separator tube for routine serum testing and keep plain red-top tubes for assays that require serum without gel. Specialty tubes should be purchased only for clearly defined applications. The safest and most economical decision comes from matching the tube to the validated laboratory method, not from selecting the most familiar color or the lowest quoted price.

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