A lithium-salt derivative of heparin, this anticoagulant sits inside evacuated blood collection tubes to keep specimens from clotting before analysis. It binds antithrombin III and accelerates that protein’s ability to inactivate thrombin and factor Xa, two central enzymes in the clotting cascade. Because the additive keeps blood from clotting after the draw, centrifugation yields plasma, a fluid that laboratories prefer over serum for many routine chemistry panels.
Green-top and light green-stoppered tubes mark this additive on most phlebotomy trays.
This walkthrough breaks down how the green-top anticoagulant works, why labs favor its plasma for routine chemistry, and how it stacks up against EDTA and sodium heparin alternatives.
Lithium Heparin as the Standard Plasma Anticoagulant
Composition and Where It Lives
Heparin is a long-chain sugar molecule, a glycosaminoglycan, that carries a strong negative charge. Pairing that molecule with a lithium counterion instead of sodium or potassium creates the specific salt used in plasma tubes. Chemistry tubes typically deliver 10 to 30 USP units of heparin per milliliter of blood, enough to keep the sample fluid from vein to analyzer without overwhelming the test chemistry downstream.
You will encounter this additive in two practical forms. Spray-dried tubes carry a thin film of heparin on the inside wall; liquid preparations deliver a small measured volume of heparin solution that mixes with blood on contact. The spray-dried version is more forgiving when a tube is filled slightly under or over its nominal draw volume, because the additive stays proportional to the wall surface rather than the liquid volume.
The Plasma Versus Serum Distinction
Plasma is the liquid component of blood that remains after cells are removed, with clotting factors preserved because an anticoagulant stopped the cascade. Serum is what is left after blood has clotted and the clot has been spun down. That difference affects every test that measures clotting proteins, fibrinogen, or anything that interacts with thrombin.
Because lithium heparin keeps clotting factors intact and gives the lab a clean cell-free liquid after centrifugation, it has become the default anticoagulant for routine clinical chemistry, emergency department panels, and most outpatient plasma biochemistry work. Sodium or potassium heparin salts can be used for the same purpose, but they introduce the very ions that electrolyte tests need to measure accurately, which is why lithium won the chemistry-tube slot.
That chemistry-driven choice also explains why lithium heparin behaves so predictably once blood enters the tube.
The Chemistry Behind Lithium Heparin’s Anticoagulant Action
The Antithrombin Pathway
The anticoagulant effect runs through a circulating protein called antithrombin III, sometimes written as ATIII. On its own, antithrombin inactivates thrombin and factor Xa slowly. Heparin binds antithrombin and speeds that inactivation roughly a thousand-fold, giving the plasma a near-instant switch for the clotting cascade. The heparin molecule acts as a template, bending antithrombin into a shape that grabs thrombin more efficiently.
Once heparin has done that job it does not stay bound. The complex diffuses away, leaving antithrombin free to bind another clotting enzyme. The heparin itself remains available to catalyze more reactions. This catalytic behavior is the reason a tiny amount, around 15 USP units per milliliter, is enough to keep a full tube of blood fluid.
Why Heparin Does Not Dissolve Existing Clots
One frequent misconception is that heparin, including the lithium salt, can break up a clot that has already formed. It cannot. Heparin only prevents new fibrin from being laid down by inhibiting the enzymes that drive clotting. To dissolve an existing clot, the body or a clinician relies on a different system, the plasminogen-to-plasmin pathway, which heparin does not affect. Once thrombin has already converted fibrinogen into a fibrin mesh, heparin stands aside.
That distinction matters at the bench. A tube drawn poorly and partially clotted before reaching the centrifuge will not be rescued by the additive. The result is fibrin strands that can clog analyzer probes and inflate certain analyte readings.
Frequent clogging and skewed readings are exactly why clinicians must decide where the tube earns its keep.
Where Lithium Heparin Tubes Fit in Clinical Testing
Routine and Plasma-Based Chemistry
Most chemistry analyzers in hospital core labs run lithium heparin plasma. A basic metabolic panel, a comprehensive metabolic panel, liver function tests, renal panels, lipid panels, and thyroid-stimulating hormone assays all commonly rely on this tube. Plasma turnaround is faster than serum because you skip the 10 to 15 minutes the blood would normally spend clotting at room temperature before centrifugation.
Plasma also gives a slightly higher yield of liquid from the same draw volume, about 15 to 20 percent more than serum, because no clotting proteins have been consumed forming a fibrin mesh. For pediatric draws or any hard-stick situation, that extra volume can matter.
Formulations and Their Handling Implications
Spray-dried lithium heparin tubes tolerate a slightly broader fill range and are commonly used in hospital admission draws. Liquid preparations are favored in some research settings and reference labs because the additive concentration can be tuned for special applications, including trace-element work or assays where the additive itself can interfere with the measurement.
Those tunable strengths and trade-offs only matter, though, when the sample itself reaches the analyzer in good shape.
For most routine chemistry panels, a standard spray-dried green-top tube drawn to its nominal volume and centrifuged within two hours gives the cleanest result.
| Formulation | Typical Use | Handling Note |
|---|---|---|
| Spray-dried on tube wall | Routine chemistry, emergency panels | Tolerates under-fill better than liquid form |
| Liquid preparation | Reference labs, specialized assays | Requires accurate fill volume for correct additive ratio |
| Plasma separator (gel barrier) | Send-out testing, delayed transport | Gel separates plasma from cells during centrifugation |
Comparing Lithium Heparin to EDTA and Sodium Heparin
Why Lithium Replaced Sodium in Chemistry Tubes
Sodium heparin was the original plasma anticoagulant for chemistry work, but it carries an obvious liability: the sodium ion from the additive itself inflates any sodium test run on the resulting plasma. Potassium heparin has the same problem in reverse. Lithium heparin solved this because lithium is rarely the target of a routine electrolyte measurement, so the additive does not interfere with the analytes the lab is trying to measure.
That single decision, lithium over sodium, is the reason every chemistry tray pulls green-top tubes rather than the older heparin formulations. It also explains why plasma lithium testing cannot use these tubes, since the additive itself would push the result artificially high.
How EDTA Differs in Purpose
EDTA, typically the dipotassium salt (K2-EDTA) inside the lavender-top tube, works by chelating calcium. Calcium is required at multiple steps of the cascade, so removing it stops clotting completely. EDTA preserves the shape and volume of blood cells, which is why hematology analyzers rely on it for complete blood counts, differential counts, and hemoglobin measurements.
The trade-off is that EDTA also strips calcium and magnesium from the plasma, so it cannot be used for any chemistry test that depends on those ions. EDTA can also distort cell morphology if the tube sits too long before analysis, and it interferes with some immunoassays, which is why chemistry and hematology tubes stayed separate on the phlebotomy tray.
| Additive | Tube Stopper | Primary Use | Key Interference |
|---|---|---|---|
| it | Green / light green | Plasma chemistry, panels | Lithium level assays, some cardiac markers |
| Sodium heparin | Green (older convention) | Plasma chemistry (legacy) | Sodium, potassium measurements |
| K2-EDTA | Lavender / pink | CBC, hematology, blood bank | Calcium, magnesium, some immunoassays |
| Sodium citrate | Light blue | Coagulation studies (PT, aPTT) | Fibrinogen, platelet function |
| Fluoride oxalate | Gray | Glucose, lactate | Sodium, some enzyme assays |
Citrate and fluoride-oxalate tubes still earn their place in specific niches. Citrate preserves clotting factors for coagulation studies because the calcium it binds can be added back during the assay. Fluoride-oxalate stops glycolysis so glucose values stay accurate when the sample will sit before analysis. Each additive solves one problem at the cost of another.
Handling Lithium Heparin Samples for Reliable Results
Mixing and Centrifugation
Once the tube is drawn, gently invert it 5 to 10 times so the heparin mixes evenly with the blood. Vigorous shaking hemolyzes red cells and releases potassium into the plasma, which then skews any potassium reading. A smooth wrist-flip is enough. Undermixing leaves a heparin-poor pocket of blood that can micro-clot, releasing intracellular contents and distorting results.
Centrifuge the tube within two hours of the draw for most chemistry assays. The plasma separator gel forms a barrier between the liquid and the cells during centrifugation, and once that barrier is set, the cells can no longer leak potassium, phosphate, or enzymes into the plasma. Delayed centrifugation is the single most common cause of spurious chemistry results on otherwise correctly drawn samples.
Storage and Transport
Plasma drawn into a separator tube can sit at room temperature for several hours after centrifugation if the assay allows. For longer holds, refrigerate the tube at 2 to 8 degrees Celsius and avoid repeated freeze-thaw cycles. Some analytes, including ammonia and lactate, are time-sensitive and need to be run on plasma separated and frozen within 30 to 60 minutes of the draw, regardless of tube type.
- Invert gently after draw: Mix 5 to 10 smooth inversions to distribute heparin without hemolyzing cells.
- Centrifuge within two hours: Spin most chemistry samples before the two-hour mark to limit potassium and enzyme leakage.
- Keep plasma separated: Use a gel-barrier tube so cells stay isolated from the liquid after centrifugation.
- Refrigerate for longer holds: Store at 2 to 8 degrees Celsius; freeze only when the assay specifically allows it.
- Avoid hemolysis: Handle smoothly and fill to the nominal line to protect potassium, LDH, and AST readings.
- Watch time-sensitive analytes: Separate and freeze ammonia and lactate plasma within 30 to 60 minutes of the draw.
Limitations and Situations Where Lithium Heparin Falls Short
Tests That Cannot Use Lithium Heparin
The clearest contraindication is any assay that measures lithium itself, typically ordered as a lithium therapeutic drug monitoring test for patients on lithium therapy for bipolar disorder. The tube additive contains lithium at concentrations far above any clinical plasma level, so the sample would report falsely elevated regardless of the patient’s status. Sodium heparin or a serum red-top tube is required.
Coagulation studies, including prothrombin time and activated partial thromboplastin time, also cannot use heparin plasma because the additive itself blocks the cascade the test is trying to measure. Light blue sodium citrate tubes are the standard for those assays because the citrate can be neutralized in the analyzer, restoring the clotting factors.
Assays Sensitive to Heparin Interference
Some immunoassays are vulnerable to heparin in the sample. High-sensitivity troponin and several point-of-care cardiac markers can read slightly low or high in heparin plasma, depending on the assay chemistry. Many labs validate which tubes are acceptable for which markers; some require a serum draw, others specifically demand it because serum adds its own variability. The analyzer insert sheet is the final word.
Some molecular and protein-based assays also struggle with heparin, which can inhibit polymerase chain reactions and certain immunoassay antibodies. Gel separator tubes have their own caveats because the gel can absorb lipophilic analytes, including some hormones and drugs. When a test seems unreliable, the question to ask is often which tube was used, not whether the patient is well.
The Big Picture
it earns its place in the modern lab because it preserves plasma without contaminating it with sodium or potassium. Its mechanism through antithrombin III explains why a small amount works, why it cannot dissolve existing clots, and why it stays out of certain assays entirely. Match the tube to the test, mix and centrifuge promptly, and the chemistry will do the rest.
FAQ
What is lithium heparin and how does it work as an anticoagulant?
it is the lithium salt of heparin, used inside evacuated blood collection tubes to prevent clotting after a blood draw. It binds antithrombin III and accelerates that protein’s ability to inactivate thrombin and factor Xa, stopping the clotting cascade in whole blood before plasma is separated.
What is lithium heparin used for in blood tests?
That standard anticoagulant for plasma-based clinical chemistry, including basic and comprehensive metabolic panels, liver and kidney panels, lipid tests, and many plasma biochemistry assays run on automated analyzers in hospitals and reference labs.
Is lithium heparin the same as heparin?
Functionally, yes; both share the same glycosaminoglycan backbone and the same antithrombin-based mechanism. The difference is the counterion: lithium is used in chemistry tubes specifically because it does not interfere with sodium and potassium measurements.
Why is lithium heparin preferred over sodium heparin for plasma samples?
Sodium heparin adds sodium ions to the plasma, distorting sodium electrolyte measurements and slightly affecting potassium readings. Lithium avoids that interference, which is why it replaced sodium heparin for routine chemistry work decades ago.
What color tube is used for lithium heparin?
Green-top tubes, and the lighter green plasma separator tubes, signal it as the anticoagulant. Stopper color varies slightly by manufacturer, but green is the universal visual cue.
Can lithium heparin affect lab test results?
It can interfere with lithium level testing, some coagulation assays, and a small number of immunoassays sensitive to heparin. Following the lab’s tube-selection guidance prevents erroneous results.
