Citrate Anticoagulant Volume Adjustment Calculator
Citrate Anticoagulant Volume Adjustment Calculator
Work out how much citrate should remain in a coagulation tube when the haematocrit is above 55%, using the CLSI H21-A5 formula — and why the same correction is not made for anaemia.
Adjusted citrate volume
Haematocrit + draw volume → mL citrateA patient with a haematocrit of 62%, drawn into a standard 5 mL citrate tube (4.5 mL of blood)
Formula
citrate to remove = citrate as supplied − C
- C
- the volume of sodium citrate, in millilitres, that should REMAIN in the tube before the blood is drawn
- HCT
- the patient’s haematocrit as a percentage, so that (100 − HCT) is the plasma fraction of the blood in percentage points
- Vblood
- the volume of blood in millilitres that will be added — the tube’s nominal DRAW volume, not the total capacity of the tube
- 1.85 × 10⁻³
- the constant that reproduces the standard 1-in-10 fill at a reference haematocrit of 40%. Setting C = 0.5 mL and V = 4.5 mL gives (100 − HCT) = 60.06, so HCT = 39.94%
- > 55%
- the haematocrit above which CLSI H21-A5 recommends the adjustment. Below it the standard tube is used unchanged
Worked example
A patient with a haematocrit of 62%, drawn into a standard 5 mL citrate tube (4.5 mL of blood)
100 − 62 = 38
1.85 × 10⁻³ × 38 × 4.5 = 0.316 mL of citrate should remain
The tube as supplied holds 0.5 mL, so remove 0.5 − 0.316 = 0.184 mL before drawing
Then fill with blood to the tube's normal blood volume
Left uncorrected, that tube would carry 58% more citrate per millilitre of plasma than intended
Citrate that should remain, for a 4.5 mL blood draw
| Haematocrit | C (mL citrate to remain) | Remove from a 0.5 mL tube | Adjustment recommended? |
|---|---|---|---|
| 30% | 0.583 | — (would need citrate ADDED) | No |
| 40% | 0.500 | 0.000 | No — this is the haematocrit the standard tube is built for |
| 45% | 0.458 | 0.042 | No |
| 55% | 0.375 | 0.125 | At the threshold; CLSI recommends adjustment ABOVE 55% |
| 60% | 0.333 | 0.167 | Yes |
| 65% | 0.291 | 0.209 | Yes |
| 70% | 0.250 | 0.250 | Yes — half the citrate has to come out |
Two ways to get the same wrong answer
| Situation | What happens to the plasma | Effect on PT and APTT |
|---|---|---|
| Haematocrit above 55% | Less plasma per millilitre of blood, so the fixed 0.5 mL of citrate is concentrated into a smaller plasma volume | Falsely prolonged |
| Standard tube under-filled | Less blood, therefore less plasma, against the same fixed 0.5 mL of citrate | Falsely prolonged — by the same mechanism |
| Haematocrit below about 35% | More plasma per millilitre of blood, so the citrate is diluted | Shortened, but the effect is smaller and no correction is made |
How laboratories handle it in practice
| Approach | How it works | Trade-off |
|---|---|---|
| Specially prepared tube | Citrate is removed from a standard tube in advance to the calculated volume, the fill line is marked, and the tube is sent to the collector | Needs the haematocrit before the draw, and a system for getting the right tube to the right patient |
| Recollect once the haematocrit is known | The full blood count flags the problem, the laboratory prepares an adjusted tube and asks for a repeat draw | A second venepuncture and a delay, but no manipulation of a filled tube |
| Syringe draw with manual adjustment | Blood is drawn by syringe and added to the adjusted tube up to a marked line | Loses the closed system; needle handling and vacuum loss are the objections to it |
| Report with a comment | Some laboratories issue the result with an interpretive comment rather than adjusting | Acceptable only where the clinical question tolerates it; the prolongation is analytical, not real |
A ratio that is wrong before the analyser sees it
A citrate tube is built around one assumption: that nine parts of blood added to one part of citrate will yield a predictable concentration of citrate in the plasma. That assumption holds because the proportion of blood that is plasma is reasonably constant. Citrate distributes only into the plasma — it does not enter red cells — so when the red cell fraction rises, the same fixed volume of citrate is dissolved in less plasma, and the plasma that reaches the analyser is more heavily anticoagulated than the method was calibrated for. Prothrombin time and activated partial thromboplastin time assays work by adding calcium back; excess citrate chelates more of that calcium, less is available to drive the reaction, and the clot forms late. The prolongation is entirely analytical. Nothing has changed about the patient’s coagulation.
CLSI H21-A5 sets the threshold at a haematocrit above 55% and gives the correction as C = 1.85 × 10⁻³ × (100 − HCT) × V, where C is the volume of citrate that should remain in the tube and V is the volume of blood to be added. The constant is not arbitrary. Put the standard tube’s own figures into it — 0.5 mL of citrate, 4.5 mL of blood — and it returns a plasma fraction of 60.06%, which is a haematocrit of 39.94%. The formula is calibrated so that the standard tube is exactly right at a haematocrit of 40%, and everything it does thereafter is a correction away from that reference point. At 45% it asks for 0.458 mL rather than 0.5, so the standard tube supplies about 9% more citrate than ideal — which is the same order as the fill variation the standard already accepts, since CLSI requires only that tubes be at least 90% full. That is why nothing is done in the normal range. By 55% the excess is a third, and by 70% the tube holds twice the citrate it should.
Two details of the formula are worth stating because both are easy to get wrong. V is the volume of blood, not the volume of the tube: for a standard 5 mL citrate tube that is 4.5 mL, and using 5.0 inflates C by about 11%. And C is the volume to leave behind, not the volume to take out — the quantity to remove is the citrate the tube was supplied with, usually 0.5 mL, minus C. The Allina Health procedure works its example the same way: a haematocrit of 60% in a 5 mL tube gives 0.33 mL of citrate remaining, so 0.17 mL is removed.
The other half of the question is what to do when the haematocrit is low, and the honest answer is that the standards do not say, because they do not address it. CLSI H21-A5 as quoted by the laboratory procedures that implement it, the Allina Health SOP, and Labcorp’s collection guidance all deal exclusively with the high side. The mechanism certainly runs in the other direction — an anaemic patient has more plasma per millilitre of blood, so the citrate is more dilute and clotting times are shortened rather than prolonged — but the magnitude is smaller, because plasma volume varies less going down from 40% haematocrit than going up, and the secondary literature reports that haematocrits down as far as 20% have no significant effect on PT or APTT. No adjustment is recommended, and a laboratory that invents one is departing from the standard rather than following it.
All of which matters less than the commonest version of the same error, which has nothing to do with haematocrit at all. An under-filled tube produces precisely the same fault by precisely the same route: less blood means less plasma against an unchanged 0.5 mL of citrate, and the result is a falsely prolonged clotting time indistinguishable from the one polycythaemia causes. Fill volume is the first thing to check when a coagulation screen is unexpectedly abnormal, and two under-filled tubes must never be combined into one, because the citrate adds up along with the blood.
Frequently asked questions
When does the citrate volume need adjusting?
When the haematocrit is above 55%. CLSI H21-A5 sets that threshold and requires laboratories to have a written procedure for such specimens. Below 55% the standard tube is used unchanged.
What does the formula give me — the citrate to remove or the citrate to keep?
The volume to keep. C is the citrate that should remain in the tube. Subtract it from the volume the tube was supplied with — usually 0.5 mL for a standard 5 mL tube — to get the volume to remove.
Is V the tube volume or the blood volume?
The blood volume. For a standard 5 mL citrate tube that is 4.5 mL, because the remaining 0.5 mL is the citrate. Entering 5.0 mL instead makes the answer about 11% too large.
Why is the constant 1.85 × 10⁻³?
Because it reproduces the standard tube at a haematocrit of 40%. Solve the equation for C = 0.5 mL and V = 4.5 mL and it returns a plasma fraction of 60.06%, a haematocrit of 39.94%. That is the reference point the ordinary 9:1 fill is built around.
Should I adjust the citrate for a low haematocrit?
No. CLSI H21-A5 and the laboratory procedures that implement it address only haematocrits above 55%. The error runs the other way in anaemia — clotting times are shortened rather than prolonged — but it is smaller, and reported to be insignificant down to a haematocrit of about 20%.
Why does a high haematocrit prolong the PT and APTT?
Citrate distributes only into plasma. A high red cell fraction means less plasma to dissolve the same fixed volume of citrate, so the plasma reaching the analyser carries more citrate, chelates more of the calcium the reagent adds, and clots late. The patient’s coagulation is unaffected.
Is an under-filled tube the same problem?
Yes, by the same mechanism: less blood means less plasma against an unchanged volume of citrate. CLSI H21-A5 requires citrate tubes to be at least 90% full, and two under-filled tubes must never be pooled, because the citrate is pooled along with the blood.
Related calculators
References
- Clinical and Laboratory Standards Institute. Collection, Transport and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays; Approved Guideline — Fifth Edition. CLSI document H21-A5, Vol. 28 No. 5.
- Allina Health Laboratory. Hematocrit-Anticoagulant Adjustments (March 2024) — the formula as implemented, the worked 60% example and the marked-tube procedure.
- CAP TODAY. Q&A column, December 2019 — the formula, the >55% threshold, and the mechanism: citrate distributes only in plasma, so a reduced plasma volume means increased calcium chelation.
- Labcorp. Blood Specimens: Coagulation — the 9:1 fill requirement and the citrate adjustment for a known haematocrit above 55%.
- Are tube fill volumes below 90% a rejection criterion for all coagulation tests? Lab Med 2024;55(4):442 — CLSI H21-A5 recommends that citrate tubes be at least 90% full, and the effect of under-filling on clot-based assays.
- The Fritsma Factor. Specimens with Hematocrit >55% and Specimen Collection: HCT over 55% — practical routes to an adjusted tube, and the observation that low haematocrit does not usually require adjustment.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.