The tube used for an erythrocyte sedimentation rate (ESR) test depends on whether your lab runs the test by hand or on an automated analyzer. In most modern laboratories, blood for ESR is drawn into a lavender-topped (purple-capped) tube containing the anticoagulant EDTA. Labs still using the classic Westergren method may instead use a black-topped tube containing sodium citrate. The distinction matters because the wrong tube can produce inaccurate results, and the field has been shifting toward one standard over the past few decades.
Two Tubes, Two Anticoagulants
Blood drawn for any lab test that needs to stay liquid goes into a tube with an anticoagulant, a chemical that prevents clotting. For ESR testing, two anticoagulants have been in wide use. The first is trisodium citrate, packaged in tubes with a black rubber stopper (sometimes called Sediplast or ESR-specific tubes). These tubes contain citrate solution pre-measured so that blood mixes at a 4:1 ratio. The second is EDTA (ethylenediaminetetraacetic acid), packaged in tubes with a lavender or purple stopper. EDTA tubes are the same ones used for a complete blood count, which makes them already familiar to every phlebotomist and present in virtually every blood-draw tray.
The color coding follows standardized conventions used across manufacturers. Lavender caps signal K₂-EDTA or K₃-EDTA. Black caps signal sodium citrate buffered for sedimentation testing (distinct from the light-blue-topped citrate tubes used for coagulation studies, which have a different citrate concentration). If you see your blood drawn into a purple-topped tube and your lab slip lists an ESR, that is normal and expected in a lab using automated ESR instruments.
Why the Field Moved Toward Lavender Tubes
The traditional Westergren ESR method, introduced in the 1920s, required diluting blood with sodium citrate, drawing it into a long glass pipette, standing the pipette upright, and reading how far the red cells had settled after one hour. The method worked well, but it demanded a dedicated citrate tube, manual pipetting of open blood, and a full 60-minute wait. Each of those steps created practical problems: exposure of lab workers to uncapped blood, a risk of aerosol generation, and a separate blood draw if the patient also needed a complete blood count.
In 1993, the International Council for Standardization in Haematology (ICSH) updated its recommendations and formally recognized EDTA-anticoagulated, undiluted blood in Westergren-length pipettes as the reference method for ESR measurement. The same document also laid out specifications for standardized working methods using either diluted or undiluted blood.
1PubMed Central. ICSH recommendations for measurement of erythrocyte sedimentation rateThat endorsement of EDTA was a turning point. Labs could now run an ESR from the same purple-topped tube already collected for a blood count, eliminating the need for a separate draw and a separate tube type. Over the following decades, as automated ESR analyzers replaced manual racks in most hospital and reference laboratories, the lavender tube became the de facto standard.
How Automated Analyzers Use the EDTA Tube
Modern automated ESR instruments accept a standard lavender-topped EDTA tube, aspirate a small volume of blood, and return a Westergren-equivalent result in roughly 20 minutes rather than the traditional 60. One widely used system, for example, measures the height of the plasma column that forms as red cells settle, using an infrared sensor to track the boundary between cells and plasma. An internal algorithm then converts the reading to a Westergren-equivalent value, adjusted for ambient temperature.
2PubMed Central. Performance Evaluation of Automated Erythrocyte Sedimentation Rate (ESR) Analyzers in a Multicentric StudyThese analyzers typically require a tube diameter of about 12 to 13 mm, which matches the standard Kâ‚‚-EDTA venipuncture tube found in most blood collection systems.
3PubMed Central. Analytical Agreement Between the Vision C Automated ESR Analyzer and the Manual Westergren Reference MethodThe practical upshot for patients is straightforward: if your doctor orders both a blood count and an ESR, both tests can often be run from a single lavender-topped tube. That means one less tube of blood drawn from your arm.
Some labs, particularly in settings with older equipment or in parts of the world where manual Westergren racks are still standard, continue to request a separate black-topped citrate tube for ESR. A comparative study of anemic patients, for example, measured ESR on an automated analyzer using EDTA blood alongside manual Westergren readings on citrate blood, demonstrating that the two approaches can coexist in the same lab.
4PubMed Central. Comparative Analysis of Erythrocyte Sedimentation Rate Measured by Automated and Manual Methods in Anaemic PatientsClosed-System Tubes for Safety
One of the longstanding complaints about the manual Westergren method was that it required lab workers to open blood tubes, pipette samples by hand, and handle open-ended glass columns. This created real biohazard exposure risks, particularly after the emergence of blood-borne infections like HIV and hepatitis C in the 1980s. In response, manufacturers developed sealed vacuum-extraction tubes specifically designed for ESR testing.
The Seditainer system, for instance, is a sealed vacuum tube that allows the ESR to be read without ever opening the specimen. An evaluation of this system found it was easy to use, had acceptable precision compared with the standard Westergren technique, and showed only a small decrease in ESR values (under 2% on average) when sealed specimens were stored overnight at 4 °C.
5PubMed Central. Evaluation of sealed vacuum extraction method (Seditainer) for measurement of erythrocyte sedimentation rateA similar product, the ESrT-system 200, uses a 215 mm long vacuum aspiration tube with pre-loaded anticoagulant diluent. Blood flows directly into the tube during venipuncture, and the sedimentation reading is taken without direct handling. This approach preserves the Westergren-length measurement column while substantially reducing biohazard risk.
6PubMed Central. Evaluation of a 200 mm long vacuum aspiration tube for measurement of erythrocyte sedimentation rateThese dedicated ESR tubes typically have their own cap color (often black or gray, depending on the manufacturer) and should not be confused with general-purpose citrate or EDTA tubes. If your lab’s requisition form specifies one of these proprietary systems, the phlebotomist will know which tube to reach for.
How Quickly the Sample Needs to Reach the Lab
Regardless of which tube your blood goes into, timing matters. Red blood cells change shape and lose their tendency to stack into rouleaux (the coin-like columns that drive sedimentation) as a sample ages. That means a delayed sample generally gives a falsely low ESR.
A study that tracked ESR changes over time found no meaningful difference from baseline when samples were tested within six hours at room temperature. Beyond eight hours at room temperature, however, ESR dropped by roughly 11%. Refrigerated samples held up somewhat better: storing blood at 4 °C preserved the ESR reading for up to eight hours with less than a 9% change, though a significant decrease appeared by 12 hours.
7PubMed Central. Effect of storage temperature and time on erythrocyte sedimentation rateA second study using a different automated analyzer confirmed the pattern and pushed the refrigerated window further. Samples kept at 4 °C showed high agreement with fresh results up to 36 hours, while room-temperature samples became unreliable past six hours.
8PubMed Central. Evaluation of Stability and Accuracy Compared to the Westergren Method of ESR Samples Analyzed at VES-MATIC 5The practical takeaway: if your blood is drawn at an outpatient clinic that ships samples to a central lab, refrigeration during transport is important for ESR accuracy. And if you are ever told your ESR is surprisingly low, ask how long the sample sat before testing, because delay alone can suppress the result.
What Doctors Are Looking for When They Order an ESR
The ESR is one of the oldest and simplest blood tests still in routine use. It measures how quickly red blood cells settle to the bottom of a vertical tube, expressed in millimeters per hour. Faster settling usually means more inflammation somewhere in the body, because inflammatory proteins in the blood cause red cells to clump together and fall faster.
Doctors order ESR most often when they suspect a chronic inflammatory condition but need a broad, inexpensive screening test to support or weaken that suspicion. Giant cell arteritis (also called temporal arteritis) is one of the classic situations where ESR plays a central diagnostic role. The American College of Rheumatology includes an elevated ESR, usually above 50 mm/h, among its classification criteria for the disease.
9PubMed Central. Temporal arteritis with erythrocyte sedimentation rate <50 mm/h: a clinical reminderThat said, ESR is not a precise tool. It rises with infection, autoimmune disease, cancer, and even normal aging. A high result tells a doctor that something inflammatory is going on; it does not say what. And a normal result does not rule out disease. In a study of 24 patients with biopsy-confirmed giant cell arteritis, some had ESR values as low as 1 to 40 mm/h at presentation, leading the researchers to conclude that clinical suspicion should carry more weight than any particular ESR cutoff.
10PubMed. Erythrocyte sedimentation rate and its relationship to hematocrit in giant cell arteritisWhen ESR Results Are Unreliable
Several conditions can push ESR readings in directions that have nothing to do with inflammation, which makes interpreting results trickier than it might seem.
Sickle cell disease is a well-known confounder. Sickled red blood cells are rigid and irregularly shaped, so they do not form rouleaux the way normal cells do. A study of 27 asymptomatic patients with sickle cell anemia found abnormally low sedimentation rates that could not be corrected by swapping in plasma from healthy donors, confirming the effect was driven by the cells themselves rather than by anything in the plasma. During a painful sickle cell crisis, ESR rose, but the increase was attributed mainly to red cell changes rather than to inflammatory proteins.
11PubMed. Erythrocyte sedimentation rate during steady state and painful crisis in sickle cell anemiaPregnancy is another situation where ESR shifts substantially even in the absence of disease. A study of over 1,000 pregnant women found ESR values ranging from 4 to 112 mm/h. In non-anemic women, the normal range climbed from roughly 18 to 48 mm/h in the first half of pregnancy to 30 to 70 mm/h in the second half. Both gestational age and hemoglobin concentration influenced the results.
12PubMed. Pregnancy and the erythrocyte sedimentation rateThis means that a pregnant woman with an ESR of 55 mm/h in her third trimester could be perfectly healthy, while the same value in a non-pregnant adult would warrant investigation. Doctors who order ESR during pregnancy need to apply pregnancy-specific reference ranges rather than the standard cutoffs.
Anemia of any cause tends to raise ESR, because fewer red cells in the tube means less resistance to sedimentation. Polycythemia (too many red cells) does the opposite, artificially lowering ESR. Obesity, kidney disease, and even high cholesterol can nudge results in one direction or another. These confounders are one reason some clinicians prefer C-reactive protein (CRP) as an inflammatory marker, though neither test is perfect on its own.
ESR Compared with CRP
CRP is the other common blood marker of inflammation, and you might wonder why doctors still bother with ESR when CRP exists. The two tests measure different things and respond on different timelines. CRP is a protein produced by the liver that rises within hours of an inflammatory stimulus and falls quickly once the stimulus resolves. ESR, by contrast, changes more slowly and is influenced by factors beyond inflammation, including the size, shape, and number of red blood cells.
In acute settings, CRP is generally the sharper tool. A prospective study of multiple trauma patients found that ESR had poor predictive performance for outcomes, while CRP, though also imperfect, tracked more closely with the severity of injury. The researchers noted that ESR changes more slowly and is confounded by non-specific factors, making it less useful for rapid triage.
13PubMed Central. Prognostic value of on-admission IL-6, lactate, base deficit, ESR, and CRP in a prospective study of multiple trauma patientsWhere ESR keeps its clinical footing is in chronic inflammatory conditions, especially those where CRP can fluctuate day to day while ESR offers a smoother, longer-term picture. In monitoring rheumatoid arthritis, lupus, or polymyalgia rheumatica over weeks and months, many rheumatologists still order both tests and interpret them side by side rather than choosing one over the other.
Micro-ESR for Newborns
Drawing a full tube of blood from a newborn is difficult and potentially harmful, given how little blood a tiny infant has. For neonatal sepsis screening, some hospitals use a micro-ESR technique that requires only a small capillary sample. This mini version of the test follows the same sedimentation principle but uses far less blood, making it practical even in resource-limited settings where expensive automated analyzers are unavailable.
14Egyptian Pediatric Association Gazette. Comparative efficacy of micro-erythrocyte sedimentation rate (m-ESR) and C-reactive protein (CRP) as a neonatal septic screening markerThe blood for a micro-ESR is typically collected by heel prick into a heparinized capillary tube rather than into a standard venipuncture tube. The tube is tiny, the read time is short (usually one hour or less), and the result can help a pediatric team decide quickly whether to start antibiotics in a newborn showing signs of infection. This method is not interchangeable with adult ESR values, so it carries its own reference ranges and interpretation guidelines specific to neonates.
Ordering Tips If You Are a Patient
If your doctor has ordered an ESR and you are heading to a blood draw, you do not need to do anything special. No fasting is required. The phlebotomist will select the correct tube based on the lab’s equipment. If you are curious about which method your lab uses, you can ask whether they run ESR on an automated analyzer (lavender tube) or manually (black tube or proprietary ESR tube). Either way, the test itself involves a routine venipuncture and produces results the same day in most labs.
One thing worth knowing: if you have blood drawn at a clinic that sends specimens out to a reference lab, the transit time could affect your ESR result. Asking whether the sample will be refrigerated during transport is a reasonable question, especially if you are being monitored for a chronic condition and small changes in ESR matter for treatment decisions. A sample that sits at room temperature for many hours before testing may come back with a falsely low number, potentially masking active inflammation.