Vancomycin is a powerful antibiotic primarily used to treat serious bacterial infections, particularly those caused by methicillin-resistant Staphylococcus aureus (MRSA). Because this medication is highly effective but potentially toxic, its concentration in the patient’s bloodstream must be precisely monitored. Understanding the correct blood collection method is essential for patient safety and accurate medication management, ensuring the laboratory test provides a true measurement of the drug’s concentration.
The Correct Collection Tube for Vancomycin Trough
The specific color of the blood collection tube for a vancomycin trough level is generally a Red Top or a Gold Top (Serum Separator Tube or SST). These tubes are selected because they are designed to yield a clean sample of serum, which is the preferred specimen type for vancomycin testing. The plain Red Top tube contains no additives, allowing the blood to clot naturally before the serum is separated during centrifugation.
Alternatively, some laboratories may accept a Green Top tube containing lithium heparin, which yields plasma, but serum is generally preferred. The most important factor is the tube’s lack of a serum-separating gel or anticoagulants that can chemically interfere with the drug measurement. For this reason, many institutions specifically reject samples collected in Gold Top (SST) tubes because the gel material can absorb vancomycin and lead to a falsely low result.
The most consistent and widely accepted tube is the plain Red Top, which contains no separator gel or interfering anticoagulant, yielding a pure serum sample. If a Red Top is used, the blood must be allowed to clot completely, and the resulting serum must be physically separated from the blood cells before analysis.
Therapeutic Drug Monitoring and the Trough Level
Monitoring the concentration of vancomycin is necessary because the drug has a narrow therapeutic window, meaning the difference between a concentration that is effective and one that is toxic is small. This process is called Therapeutic Drug Monitoring (TDM) and it helps clinicians balance the drug’s effectiveness against its potential for harm. If the drug level is too low, the infection may not be cleared, and the bacteria could develop resistance to the antibiotic.
The vancomycin “trough level” is the lowest concentration of the drug found in the patient’s bloodstream during a dosing cycle. This measurement is taken immediately before the next scheduled dose is administered, often within 30 minutes of the next dose. Measuring the trough is a surrogate marker for the overall drug exposure and helps confirm that the concentration has not dropped below the minimum effective concentration required to kill the target bacteria.
Maintaining an adequate trough level is important for preventing the development of drug-resistant organisms. If the trough concentration is too high, the patient is at an increased risk for serious side effects, most notably nephrotoxicity (damage to the kidneys). TDM ensures the patient receives a dose high enough to eradicate the infection but low enough to avoid organ damage. Monitoring serum creatinine levels is often performed alongside vancomycin TDM to watch for early signs of kidney injury.
How Tube Additives Interfere with Test Results
The specific additives present in various blood collection tubes can significantly interfere with the laboratory assays used to measure vancomycin concentration. The most common method for measuring vancomycin is an immunoassay, which relies on a precise chemical reaction between the drug and a testing reagent. Any substance in the blood sample that disrupts this reaction can lead to an inaccurate result.
Tubes containing separating gels, such as the Gold Top or Tiger Top (SST), are often rejected because the gel material can physically absorb a portion of the vancomycin. This absorption lowers the measured concentration, leading to a falsely low trough result and potentially causing a clinician to increase the patient’s dose unnecessarily. Additionally, certain anticoagulants, like EDTA found in Lavender Top tubes, can chemically interfere with some immunoassay platforms.
Inaccurate results, whether falsely high or falsely low, directly endanger patient safety because dosing decisions rely on the reported concentration. A falsely high result could lead to withholding the drug, allowing the infection to progress, while a falsely low result could lead to an overdose and increased risk of toxicity. Therefore, using a plain tube, such as a Red Top, which ensures a sample free of common interfering agents, is crucial for accurate therapeutic drug monitoring.