A vancomycin trough is drawn into a red-top tube (plain, no additive) or a gold-top serum separator tube (SST), depending on your laboratory’s specific protocol. Both collect clotted blood that yields serum, which is the specimen type most vancomycin assay platforms are validated to measure. Some laboratories also accept a green-top tube containing lithium heparin, which produces plasma instead of serum. The tube choice is not arbitrary; it affects how the sample behaves on the analyzer and whether the result can be trusted. But the tube is only one piece of getting a reliable vancomycin level, and much of what goes wrong with these draws has nothing to do with the tube color at all.
Why Serum Tubes Are the Default
Most hospital laboratories run vancomycin levels on immunoassay analyzers calibrated for serum specimens. Serum is the liquid portion of blood after it has been allowed to clot and the clot is spun down. Red-top tubes contain no additive and rely on the blood clotting naturally against the glass or silica-coated interior. Gold-top SSTs work on the same principle but include a gel layer that migrates between the clot and the serum during centrifugation, creating a physical barrier. That barrier makes it easier to pour off or pipette the serum without disturbing the clot, and it keeps the serum stable for a longer window.
A study evaluating Sarstedt serum tubes with and without separator gel found no clinically significant differences for a range of therapeutic drugs on the day of collection, and samples remained stable for up to 72 hours when refrigerated at 4°C.1PubMed. Influence of separator gel in Sarstedt S-Monovette® serum tubes on various therapeutic drugs, hormones, and proteins This matters in practice because vancomycin troughs are often drawn in the middle of the night and may sit in a transport bin for hours before reaching the analyzer. A gel barrier helps preserve the sample’s integrity during that wait.
Green-top lithium heparin tubes are an alternative at some institutions. Heparin prevents clotting, so the sample yields plasma rather than serum. The practical advantage is speed: plasma tubes do not require a clotting period before centrifugation, which shaves time off the turnaround. The disadvantage is that not every vancomycin assay is validated for heparin plasma, and certain heparin formulations can interfere with specific analyzer platforms. If your lab accepts green-tops for vancomycin, that is because they have verified compatibility with their particular instrument. If you are unsure, the safe default is always a red-top or gold-top.
Tubes You Should Not Use
Lavender-top (EDTA) tubes, blue-top (sodium citrate) tubes, and gray-top (sodium fluoride/potassium oxalate) tubes are not appropriate for vancomycin levels. EDTA and citrate are anticoagulants designed for hematology and coagulation testing, respectively. Their additives can interfere with the immunoassay chemistry used to measure vancomycin, producing unreliable results. Gray-tops are intended for glucose testing and contain preservatives that alter the sample matrix in ways that vancomycin assays are not built to handle.
A less obvious mistake is grabbing a serum tube that already contains another additive, such as a tube with thrombin accelerant meant for stat chemistry panels. While these produce serum, the accelerant or other additives may not have been tested against your lab’s vancomycin method. When in doubt, check the laboratory’s own test directory, which specifies exactly which tube types are acceptable for each analyte. Most hospitals publish this internally and update it when they change analyzer platforms.
Timing Matters More Than the Tube
The single biggest source of error in vancomycin trough monitoring is not the tube color but when the blood is drawn. A trough level is supposed to represent the lowest concentration of drug in the bloodstream, which occurs just before the next scheduled dose. Drawing the sample even 30 to 60 minutes too early can catch the tail end of the distribution phase, when vancomycin is still moving from the blood into tissues, yielding a number that looks higher than the true trough.
One study examining the timing of vancomycin trough draws found that roughly 41% of specimens were collected too early. Those mistimed samples produced average concentrations significantly higher than correctly timed ones, and clinicians responded by reducing, holding, or discontinuing vancomycin doses more often than they did with properly drawn troughs.2American Journal of Clinical Pathology. What Proportion of Vancomycin Trough Levels Are Drawn Too Early? Frequency and Impact on Clinical Actions The downstream effect is real: patients may end up underdosed for serious infections because their lab result was artificially inflated by poor timing.
A separate analysis confirmed that adherence to timing guidelines was inconsistent across healthcare settings, reinforcing that this is a systemic issue rather than a rare slip-up.3PubMed Central. Implications of Vancomycin Troughs Drawn Earlier Than Current Guidelines If you are a nurse, phlebotomist, or provider ordering the draw, the timing window is typically within 30 minutes before the next dose. Labeling the specimen with the exact time of the draw and the time the next dose is due gives the pharmacist or physician the context they need to interpret the result correctly, even if the timing was slightly off.
Why Vancomycin Levels Are Monitored So Closely
Vancomycin is used to treat serious infections caused by resistant bacteria, and it has a narrow therapeutic window. Too little drug and the infection persists; too much and the kidneys take damage. The relationship between elevated trough concentrations and kidney injury is well-documented. A meta-analysis evaluating the link between vancomycin trough levels and acute kidney injury confirmed that higher troughs carry a greater risk of renal impairment, making the accuracy of each measurement clinically important.4Journal of Antimicrobial Chemotherapy. Relationship of vancomycin trough levels with acute kidney injury risk: an exposure–toxicity meta-analysis
This is part of why the field has been shifting away from relying on a single trough number. The 2020 consensus guidelines from ASHP, IDSA, and other professional pharmacy and infectious disease organizations recommended moving toward monitoring based on the area under the concentration-time curve (AUC) rather than targeting trough levels alone. The older 2009 guidelines had recommended targeting troughs of 15 to 20 mg/L as a practical shortcut, but subsequent evidence showed that hitting those targets was associated with increased kidney toxicity without a clear gain in infection outcomes.5American Journal of Health-System Pharmacy. ASHP/PIDS/SIDP/IDSA Revised Consensus Guideline and Review for Therapeutic Monitoring of Vancomycin for Serious Methicillin-Resistant Staphylococcus aureus Infections A systematic review and meta-analysis comparing AUC-guided dosing to trough-only dosing confirmed a reduced risk of acute kidney injury with the AUC approach at similar levels of effectiveness.6PubMed Central. Vancomycin area under the curve versus trough only guided dosing and the risk of acute kidney injury: Systematic review and meta-analysis
Despite the guideline shift, many hospitals still order trough levels, either because they have not yet adopted AUC-based monitoring software or because troughs remain part of their local dosing protocol. If your institution still uses trough-based monitoring, the tube selection and timing guidance remain directly relevant. And even under AUC-guided protocols, blood samples still need to be collected in the correct tubes, typically one or two draws at specified times rather than a single pre-dose trough.
How the Lab Actually Measures Vancomycin
Once your sample reaches the lab, vancomycin concentration is almost always measured by immunoassay. These are automated methods that use antibodies to detect the drug in serum or plasma. The most common platforms include techniques called CMIA, KIMS, EMIT, and FPIA, which differ in their detection chemistry but all produce a vancomycin concentration in micrograms per milliliter. A comparison of multiple immunoassay methods against a reference technique found that most performed within about 10% of the reference value, though one method showed a difference greater than 20%, leading to clinically discordant interpretations in up to about a fifth of samples.7PubMed. Novel LC-MS/MS method for plasma vancomycin: comparison with immunoassays and clinical impact
A study in patients on hemodialysis found that two widely used commercial immunoassays both correlated well with the reference method, though one read slightly lower on average than the other.8Brazilian Journal of Infectious Diseases. Comparison of vancomycin assays in patients undergoing hemodialysis The practical takeaway is that switching between hospitals or laboratories can produce small but meaningful differences in your reported vancomycin level, even if the sample was drawn perfectly. Pharmacists who manage vancomycin dosing are generally aware of this, but it is worth flagging if a patient is transferred mid-course and their “new” level looks unexpectedly different from the last one.
When Lab Results Cannot Be Trusted
Immunoassays are fast and convenient, but they have a known vulnerability: interference from abnormal proteins in the blood. One documented case involved a patient whose blood sample returned a vancomycin concentration of 36.1 mg/L before they had ever received a single dose. The result stayed falsely elevated over months. When the same sample was tested by a reference method, vancomycin was undetectable. Investigation traced the interference to an endogenous protein in the patient’s blood that fooled the immunoassay.9PubMed. Falsely Elevated Vancomycin Concentrations in a Patient Not Receiving Vancomycin
A related report described falsely elevated vancomycin levels on the KIMS immunoassay platform due to interference from a monoclonal immunoglobulin (a type of abnormal protein seen in certain blood cancers). The analyzer flagged the result with a “kinetic flag” error, which tipped off the laboratory to investigate further.10PubMed. Resolving falsely elevated vancomycin levels in KIMS immunoassay due to monoclonal IgM interference These cases are uncommon, but they underline an important point: a vancomycin level that does not match the clinical picture, such as a high reading in a patient who seems to be clearing the drug normally, should prompt the team to consider assay interference rather than reflexively adjusting the dose. Labs can resolve ambiguous cases by retesting the sample with a different method or treating it to remove interfering proteins before re-analysis.
Monitoring Vancomycin in Patients on Dialysis
Patients with kidney failure on hemodialysis present a unique challenge for vancomycin monitoring. Vancomycin is cleared partly by the kidneys, so people with little or no kidney function accumulate the drug more slowly between dialysis sessions, and the dialysis procedure itself removes a variable amount depending on the machine and filter used. Multiple pharmacokinetic factors complicate interpretation, including a prolonged distribution phase, a rebound in vancomycin concentration after dialysis ends, differences in patient weight, residual kidney function, and how much drug is cleared by non-kidney routes.11American Journal of Health-System Pharmacy. Vancomycin dosing and monitoring for patients with end-stage renal disease receiving intermittent hemodialysis
Tube selection does not change for dialysis patients: a red-top or gold-top tube is still the standard. What changes is the timing of the draw relative to the dialysis session. Drawing blood immediately after dialysis captures a transiently low concentration that does not reflect what the level will rebound to over the following hours. Many protocols call for pre-dialysis levels or levels drawn at a standardized time after the session ends, but practices vary widely across institutions. This is an area where close communication between the pharmacy, the dialysis unit, and the nursing staff collecting the sample is essential.
Microsampling and Alternative Specimen Types
Researchers have explored alternatives to traditional venipuncture for vancomycin monitoring, particularly in pediatric patients where drawing large blood volumes is difficult. One validated method uses a microsampling device that collects just 20 microliters of whole blood from a finger or heel stick, absorbing it onto a small sponge-like tip that can be shipped to a reference lab for analysis.12PubMed Central. A whole blood microsampling assay for vancomycin: development, validation and application for pediatric clinical study This approach uses whole blood rather than separated serum, so conventional tube-color rules do not apply. The technology is not yet standard in routine clinical practice, but it has been validated and used in pediatric studies, and it offers a potential path toward less invasive monitoring in vulnerable populations.
Saliva has also been investigated as a non-invasive alternative. However, a study measuring vancomycin concentrations in both plasma and saliva found only a weak correlation between the two, concluding that saliva is not a suitable specimen type for vancomycin monitoring.13PubMed. Therapeutic Drug Monitoring of Vancomycin in Jordanian Patients. Development of Physiologically-Based Pharmacokinetic (PBPK) Model and Validation of Class II Drugs of Salivary Excretion Classification System (SECS) For now, venous blood collected in the correct serum or approved plasma tube remains the only reliable specimen for clinical vancomycin monitoring.
A Practical Checklist for Getting It Right
Given how many things can go wrong between the order and the result, a simple mental checklist helps. Before drawing the specimen, confirm the scheduled time of the next vancomycin dose and aim to draw the blood within 30 minutes before it. Use the tube your laboratory specifies. In the absence of specific guidance, a gold-top SST or red-top plain tube is the safest choice. Label the specimen with the exact draw time and the time the next dose is due. If using a red-top, allow sufficient clotting time (usually 20 to 30 minutes) before sending the sample for centrifugation.
After the result comes back, context matters as much as the number itself. A trough level that seems surprisingly high in a patient who appears clinically stable warrants a check on timing: was the sample truly drawn at the trough, or was it pulled early? If timing checks out and the result still does not match the clinical picture, assay interference is worth considering, especially in patients with known blood cancers or abnormal protein levels. Pharmacists managing vancomycin dosing rely on accurate, well-timed samples to make decisions that directly affect both infection control and kidney safety, so every step from tube selection through labeling contributes to the final clinical outcome.