What Tube Is Used for an ESR Test?

The tube used for an erythrocyte sedimentation rate (ESR) test depends on the method your laboratory uses. The traditional Westergren method calls for a black-top tube containing trisodium citrate as the anticoagulant, with blood drawn to a specific fill line so the ratio of citrate to blood stays correct. However, many modern labs now run ESR tests on lavender-top (purple-top) EDTA tubes, the same tubes used for a complete blood count, thanks to automated analyzers that have changed what is needed at the point of collection. Understanding which tube your lab requires matters because the wrong anticoagulant or fill volume can produce inaccurate results.

The Traditional Black-Top Citrate Tube

The Westergren method, introduced in 1921, remains the reference standard for ESR measurement worldwide. It requires a blood sample mixed with trisodium citrate at a four-to-one ratio: 1.6 milliliters of blood combined with 0.4 milliliters of citrate solution in a dedicated tube.¹ The citrate works by binding calcium ions in the blood, which prevents clotting while allowing red blood cells to settle naturally under gravity in an upright tube. After one hour, the distance the red cells have fallen (measured in millimeters) gives the ESR value.

Black-top citrate tubes are pre-filled with a precise volume of liquid citrate, so they must be drawn to the marked fill line. Underfilling changes the citrate-to-blood ratio and artificially alters the result. Overfilling does the same in the opposite direction. This sensitivity to fill volume is one of the practical drawbacks of the citrate method: a short or difficult blood draw can render the sample unusable.

Closed vacuum-extraction systems like the Seditainer were developed partly to address biohazard concerns with the original open-tube Westergren method. The Seditainer sealed the sample from collection through reading, reducing exposure risk for lab staff while maintaining acceptable precision.² These sealed citrate tubes are still in use in many clinical settings.

The Shift to Lavender-Top EDTA Tubes

Over the past two decades, laboratories have increasingly moved toward running ESR tests on EDTA-anticoagulated blood collected in standard lavender-top tubes. This shift was driven by automated ESR analyzers designed to read directly from EDTA tubes, eliminating the need for a separate citrate draw. The Ves-Matic Cube 200, for example, is a fully automated analyzer that performs ESR measurement using the standard EDTA blood sample tube, which markedly reduces analytical time and avoids requiring an extra blood sample from the patient.³ The Alcor iSED analyzer similarly uses primary lavender-top collection tubes for rapid automated ESR analysis.⁴

The practical benefit for patients is straightforward: if your lab uses one of these analyzers, the same blood tube drawn for your CBC can also provide your ESR result. That means one fewer tube of blood and one fewer needle stick. For labs processing high volumes, it also simplifies workflow because staff do not need to manage a separate citrate tube line. A study comparing the Roller 20LC automated analyzer (using EDTA blood) against the manual Westergren method (using citrated blood) in over 1,300 anemic patients illustrates how widespread this parallel approach has become in clinical practice.⁵

Do EDTA and Citrate Tubes Give the Same Results?

Not exactly, and this is worth knowing if you are comparing ESR values across different labs or different time periods. A comparative study among 70 patients found that ESR values were somewhat higher in EDTA-anticoagulated blood than in citrate-anticoagulated blood, with a mean difference of roughly 7 mm/h that was statistically significant.¹ The EDTA samples averaged about 58 mm/h while the citrate samples averaged about 51 mm/h in the same patients.

The mechanism behind this difference relates to how each anticoagulant interacts with plasma proteins and red blood cell behavior. Citrate dilutes the blood sample (since the liquid citrate is added to the tube), which lowers the concentration of proteins like fibrinogen that promote red cell clumping. EDTA is a dry additive that does not dilute the sample, so protein concentrations remain closer to their in-vivo levels. The result is that red cells tend to stack and settle a bit faster in EDTA blood.

For clinical purposes, automated analyzers that use EDTA tubes are calibrated and validated against the Westergren reference method, so labs report results on a comparable scale. But if your doctor is tracking your ESR over time, switching between a lab that uses citrate and one that uses EDTA could introduce a small artifact. It is always better to have serial ESR measurements done at the same lab using the same method when trends matter for your care.

How the Westergren Method Became the Reference Standard

The fundamental physics of the ESR test have been understood for over a century, and Westergren’s 1921 method proved to be the most reproducible way to perform it. A 200 mm column of citrated blood, left vertical for exactly 60 minutes, became the gold standard. Interest in ESR methodology since then has largely focused on developing automated closed systems that allow the sedimentation rate to be determined inside the collection tube, which offers advantages of speed, safety, and more uniform specimen handling.⁶ One caveat raised early on is that systems using sedimentation columns shorter than 200 mm may be less sensitive to changes at higher ESR values than the full-length Westergren column.⁶ This is something labs account for during validation, but it explains why high ESR values occasionally differ between methods.

Sample Handling After Collection

Whichever tube is used, how quickly the sample gets to the analyzer and at what temperature it is stored both influence accuracy. At room temperature, ESR values start drifting noticeably within hours. One large evaluation of 635 samples found that ESR results remained highly correlated with the baseline reading for the first four hours after collection, but accuracy dropped sharply beyond six hours at room temperature. By 24 hours at room temperature, the correlation had collapsed to the point where results were essentially meaningless.⁷

Refrigeration extends the usable window considerably. The same study showed that samples stored at 4°C maintained strong correlation with the initial reading at 24 hours and even at 36 hours, making cold storage a practical option when immediate testing is not possible.⁷ There is an important catch, though: refrigerated samples must be brought back to room temperature before testing. Running a cold sample will produce a falsely low ESR because plasma viscosity increases at cooler temperatures, slowing red cell sedimentation.⁸ If the lab has stored your sample in a refrigerator, it should be warmed to room temperature as rapidly as possible before analysis. Labs that do not follow this step can underestimate your ESR, and the error can be clinically significant for patients being monitored for inflammatory conditions.

The Seditainer vacuum system showed particularly good cold-storage performance, with ESR values decreasing by a mean of only about 2% after 24 hours at 4°C.² The sealed tube design likely helps by preventing evaporation and minimizing gas exchange that can alter pH and, in turn, red cell behavior.

Why Tube Angle Matters

An ESR tube is supposed to sit perfectly vertical during the sedimentation period. Even a slight tilt accelerates the settling of red cells and inflates the reading. Research on this effect shows that the sedimentation rate increases roughly proportionally with the sine of the tilt angle.⁹ In practical terms, a tube leaning just a few degrees from vertical can give a noticeably higher result. This is why proper ESR tube racks with firm vertical slots exist, and why the test should never be performed in a tube propped casually against something on a bench.

Interestingly, researchers have explored whether deliberate tilting could be used to speed up the test. A study comparing ESR tubes positioned at 45 degrees versus the standard 90-degree vertical position found that readings taken at the eighth minute at 45 degrees agreed well with the standard one-hour vertical reading, with minimal bias in both pregnant women and healthy blood donors.¹⁰ Readings taken earlier than the sixth minute at 45 degrees were significantly different from the reference, so timing matters if this shortcut is used. This approach is not part of standard laboratory practice, but it illustrates how sensitive the test is to physical positioning.

Pediatric and Micro-Sample Collection

Drawing enough blood for a standard ESR tube can be difficult in newborns and very small children. The conventional Westergren method needs a relatively large sample, and filling a citrate tube to its required line from tiny veins is often impractical. This is where the micro-ESR method comes in. Instead of a standard vacutainer, a small amount of blood (often just a capillary sample from a heel stick) is drawn into a narrow capillary tube, and the sedimentation is read at shorter intervals.

Micro-ESR results correlate well with conventional Westergren values. One evaluation of 108 patients confirmed a good statistical agreement between the two methods using Bland-Altman analysis.¹¹ A separate study in neonates and small children used a very small amount of blood in an EDTA tube, bypassing the need to mix with sodium citrate entirely, and found good correlation with the Westergren reference method.¹² For pediatric patients, then, the “tube” may be a small EDTA collection tube paired with a capillary tube for the actual reading, and the method offers a fast, safe determination using very little blood.

When Your Lab Order Says “ESR” — What Happens at the Draw

If your doctor orders an ESR, the phlebotomist will know which tube your lab’s analyzer requires. You do not need to specify the tube type yourself. Still, it helps to understand the possibilities so you know what to expect and can ask informed questions if something seems off.

Black-top tube: Your lab uses the traditional Westergren method or a closed-system citrate method. The tube is pre-filled with liquid citrate and must be filled to the line. You may see the phlebotomist draw this tube after other tubes in the sequence to ensure a clean, full draw.
Lavender-top tube: Your lab uses an automated analyzer that runs ESR from EDTA blood. This is the same tube used for a CBC, so if both tests are ordered, only one lavender tube may be needed.
Capillary tube: Used in neonates or when venous access is limited. The blood is drawn by finger stick or heel stick and read in a small bore tube.

Regardless of tube type, gentle mixing immediately after the draw is important. The tube should be inverted several times (not shaken vigorously) to ensure the anticoagulant contacts all the blood. Clots in the sample will invalidate the test.

Veterinary ESR Testing

ESR is not just a human clinical test. Veterinarians use it in dogs, cats, and other animals as a nonspecific marker of inflammation, just as in human medicine. The challenge in animals is similar to the pediatric challenge in humans: drawing enough blood for a standard tube can be difficult, especially in small patients, and running a separate dedicated ESR tube alongside routine blood work is wasteful.

The MINI-PET analyzer was developed specifically for veterinary use. It runs a modified Westergren method on the same EDTA-anticoagulated tube required for an automated complete blood count, so the veterinary clinic does not need multiple samples from the animal.¹³ This single-tube approach is especially valuable in cats and small dogs, where blood volume limits what can be collected safely. The broader trend in veterinary medicine mirrors the human lab shift: away from dedicated citrate ESR tubes and toward leveraging the EDTA tube that is already being drawn for hematology panels.

Common Mistakes That Affect Your Results

Beyond tube choice, a handful of pre-analytical errors can silently distort ESR values. These are worth knowing because they can lead to falsely elevated or falsely low readings that change clinical decisions.

Underfilling a citrate tube: Too much citrate relative to blood causes a falsely low ESR. If your blood draw was difficult and the tube did not fill to the line, the lab should reject it rather than run it.
Delayed testing at room temperature: As discussed, accuracy drops substantially after six hours. If your sample sits on a counter all day, the reported ESR is unreliable.
Running a cold sample: A refrigerated tube that goes straight into the analyzer without warming will underestimate the ESR.
Tilted tube: Even a few degrees off vertical inflates the reading. Automated analyzers eliminate this issue by controlling tube position mechanically, but manual Westergren setups depend on a properly designed rack.
Vibration: Setting up the test on a surface that vibrates (near a centrifuge, for example) can accelerate sedimentation and produce a falsely high value.

These errors are the lab’s responsibility, not yours as a patient. But if your ESR result seems inconsistent with your clinical picture, asking your doctor whether a repeat test at the same lab is warranted can be a reasonable step. An isolated ESR value that does not match the rest of your clinical data may reflect a handling issue rather than a real change in your inflammatory status.

Why Labs Still Use Both Tube Types

Given the convenience of EDTA tubes and automated analyzers, you might wonder why any lab still bothers with citrate tubes at all. The answer is partly institutional inertia and partly technical. The Westergren method using citrated blood remains the internationally recognized reference standard, and some regulatory or accreditation frameworks still require labs to offer it or validate their alternative methods against it. Smaller labs and clinics in resource-limited settings may also lack the capital for automated ESR analyzers and instead rely on the low-cost manual Westergren technique, which requires nothing more than a graduated glass tube, a rack, and a timer.

There is also the sensitivity question at the high end of the ESR range. Automated systems using shorter sedimentation columns may compress results at very high ESR values compared to the full 200 mm Westergren column, as noted in earlier reviews of the technology.⁶ In practice this is rarely a clinical problem since an ESR of 90 versus 100 does not usually change management, but it does mean the two methods are not perfectly interchangeable in every clinical scenario. Labs that serve rheumatology or oncology clinics, where very high ESR values are common and clinically meaningful, may be more cautious about switching away from the full Westergren method.

For most patients, the tube type is invisible: your lab handles it, and the result appears on your chart in the same millimeters-per-hour units regardless of method. The main thing worth remembering is that if you are tracking ESR over time for a condition like rheumatoid arthritis, polymyalgia rheumatica, or temporal arteritis, consistency in method and lab matters more than which tube is “better” in the abstract.

1
PubMed Central. A comparison of erythrocyte sedimentation rates of bloods anticoagulated with trisodium citrate and EDTA among TB presumptive patients at the University of Gondar comprehensive specialized hospital, northwest Ethiopia
2
PubMed. Evaluation of sealed vacuum extraction method (Seditainer) for measurement of erythrocyte sedimentation rate
3
PubMed. Evaluation of Ves-Matic Cube 200–an automated system for the measurement of the erythrocyte sedimentation rate
4
American Journal of Clinical Pathology. Comparison of Erythrocyte Sedimentation Rate (ESR) Between the Alcor iSED ESR Analyzer and the Streck Autoplus ESR Analyzer
5
PubMed Central. Comparative Analysis of Erythrocyte Sedimentation Rate Measured by Automated and Manual Methods in Anaemic Patients
6
PubMed. The mystique of the erythrocyte sedimentation rate. A reappraisal of one of the oldest laboratory tests still in use
7
PubMed Central. Evaluation of Stability and Accuracy Compared to the Westergren Method of ESR Samples Analyzed at VES-MATIC 5
8
PubMed Central. Effect of storage temperature and time on erythrocyte sedimentation rate
9
PubMed. Erythrocyte sedimentation rate II. Effects of tilt angle in saline solution
10
PubMed Central. Rapid Measurement of Erythrocyte Sedimentation Rate Using a Tube Positioned at a 45° Angle Compared With the Westergren Reference Method
11
PubMed Central. Erythrocyte Sedimentation Rate Measurement Using as a Rapid Alternative to the Westergren Method
12
Journal of Medical Science and Clinical Research. Erythrocyte Sedimentation Rate, Measurement by Capillary Tube Method, (MICRO ESR)- Best Method for Neonate and Small Children
13
PubMed Central. The Erythrocyte Sedimentation Rate (ESR) in Veterinary Medicine: A Focused Review in Dogs and Cats