From: Elizabeth M. Van Cott, M.D., and Michael Laposata, M.D., Ph.D., "Coagulation." In: Jacobs DS et al, ed. The Laboratory Test Handbook, 5th Edition. Lexi-Comp, Cleveland, 2001; 327-358.
Index of Tests
Heparin Antifactor Xa Assay [CO005000]
Synonyms Antifactor Xa Assay; Anti-Xa Assay; Heparin Assay
Applies to Danaparoid; Heparin; LMWH; Low-Molecular Weight
Heparin; Orgaran®; PF4; Platelet Factor 4; PTT
Abstract Two relatively new anticoagulants, low-molecular
weight heparin (LMWH) and danaparoid (Orgaran®), when present
at therapeutic levels, usually do not significantly prolong the
activated partial thromboplastin time (PTT). Therefore, when laboratory
tests are used to monitor therapeutic anticoagulant levels of LMWH
or danaparoid, antifactor Xa assays are necessary. In addition,
in some instances the PTT cannot be used to monitor unfractionated
heparin. For example, lupus anticoagulants* or certain factor deficiencies
(eg, factor XII deficiencies) may prolong the baseline PTT and/or
accentuate the PTT prolongation when heparin is added. In these
cases, unfractionated heparin may be monitored with antifactor Xa
assays (*Note: if the antifactor Xa assay demonstrates that
the heparinized PTT is not affected by the lupus anticoagulant,
cautious use of the PTT may be tried in that patient).
Container One blue top (sodium citrate) tube
Sampling Time Draw specimen 4 hours after subcutaneous injection
of LMWH or 6 hours after subcutaneous injection of danaparoid, otherwise,
falsely low values may occur. The therapeutic antifactor Xa ranges
with subcutaneous LMWH and danaparoid are defined for the peak levels.
Collection Routine venipuncture. Deliver tube to laboratory
immediately, otherwise falsely low values may occur (because platelets
release platelet factor 4 (PF4) which can neutralize heparin, LMWH,
or danaparoid). If multiple tests are being drawn, draw blue top
tubes after any red top tubes but before any lavender top (EDTA),
green top (heparin), or gray top (oxalate/fluoride) tubes. Immediately
invert tube gently at least 4 times to mix. Tubes must be appropriately
Storage Instructions Separate plasma from cells as soon as
possible, ideally within 1 hour of specimen collection. Otherwise,
falsely low values may occur (because platelets release PF4, which
can neutralize heparin, LMWH, or danaparoid). Plasma can be stored
for 2 hours at room temperature or on ice; otherwise, store frozen.
Turnaround Time 1 day, unless testing is batched less frequently
Special Instructions Notify the laboratory specifically as
to which drug should be measured (heparin, LMWH, or danaparoid),
because the laboratory must construct a standard curve using the
same drug that the patient is receiving.
Patients not on anticoagulants: 0 units/mL
Therapeutic range for treatment of existing deep venous
* heparin 0.3-0.7 units/mL
* LMWH: 0.4-1.1 units/mL for twice daily subcutaneous dosing.
For once daily subcutaneous LMWH dosing, the therapeutic range
is less certain but is approximately 1-2 units/mL.
* danaparoid: 0.5-0.8 units/mL
Target range for deep vein thrombosis (DVT) prophylaxis (prevention):
There is no defined target range for prophylaxis of deep
vein thrombosis (DVT) because such anticoagulation is not usually
monitored. When anti-Xa levels have been measured, mean values have
been <0.45 units/mL.
Use Determine if the patient is at the desired level of anticoagulation
with therapeutic doses of heparin, LMWH, or danaparoid
Limitations More expensive and less readily available than
the PTT for heparin monitoring
Methodology Chromogenic.1 Patient plasma is added
to a known amount of excess factor Xa with excess antithrombin.
If heparin (or LMWH or danaparoid) is present in the patient plasma,
it will bind to antithrombin and inhibit factor Xa. The amount of
residual factor Xa is inversely proportional to the amount of heparin
in the plasma. The amount of residual factor Xa is detected by adding
a chromogenic substrate that resembles the natural substrate of
factor Xa. Factor Xa cleaves the chromogenic substrate, releasing
a colored compound that can be detected by a spectrophotometer.
Results are reported as anticoagulant concentration in antifactor
Xa units/mL, such that high antifactor Xa values indicate high levels
of anticoagulation and low antifactor Xa values indicate low levels
of anticoagulation. Deficiencies of antithrombin in the patient
do not affect the assay, because excess antithrombin is provided
in the reaction.
Additional Information Therapeutic doses of unfractionated
heparin require intense laboratory monitoring, because the amount
of in vivo anticoagulation for a given dose is variable.
That is, the dose-response for heparin is unpredictable. In contrast,
LMWH and danaparoid do have a predictable dose-response, therefore,
laboratory monitoring is usually not essential. In fact, if a LMWH
or danaparoid antifactor Xa level is subtherapeutic, the most common
causes are drawing the specimen at the wrong time (see below) or
specimen transportation was longer than 2 hours. Most of the time,
LMWH and danaparoid antifactor Xa levels are in the appropriate
range when specimens are drawn correctly. Occasions in which periodic
monitoring of LMWH might be considered include renal failure, pregnancy
(increased dosage requirement in the third trimester), pediatric
patients (increased dosage requirement in newborns), obesity, underweight
patients, prolonged use, or patients at high risk for bleeding or
thrombosis. It is probably also advisable to periodically monitor
danaparoid in these same conditions.
The dose-response for unfractionated heparin is unpredictable because
many of the heparin chains are long. The long chains can bind nonspecifically
to a variety of proteins and cells, and the amounts of these heparin-binding
proteins in particular vary considerably among patients, and even
vary within the same patient at different times. In contrast, LMWH
and danaparoid consist of shorter chains (ie, low-molecular weight)
that have much less nonspecific binding.
Causes of subtherapeutic antifactor Xa level:
* specimen drawn at incorrect time (collection times are
4 hours after injection of LMWH, 6 hours after injection of
* specimen transportation longer than 2 hours
* patient receiving prophylactic dose, therefore, therapeutic
range is not applicable and anti-Xa level is actually appropriate
* higher dose needed (uncommon with LMWH or danaparoid, more
common with heparin, eg, an acute phase state often increases
the heparin dose requirement)
Causes of supratherapeutic antifactor Xa level:
* renal failure (with LMWH or danaparoid) (decreased renal
* heparin contamination, if specimen was drawn from a line
* lower dose needed (uncommon with LMWH or danaparoid, more
common with heparin)
1. Teien AN and Lie M, "Evaluation of an Amidolytic Heparin Assay
Method: Increased Sensitivity by Adding Purified Antithrombin III,"Thromb
Res, 1977, 10:399-410.
Hirsh J, Warkentin TE, Raschke R, et al, "Heparin and Low-Molecular
Weight Heparin. Mechanisms of Action, Pharmacokinetics, Dosing Considerations,
Monitoring, Efficacy and Safety,"Chest, 1998, 114(5 Suppl):489-510.
Laposata M, Green D, Van Cott EM, et al, "College of American Pathologists
Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy.
The Clinical Use and Laboratory Monitoring of Low-Molecular Weight
Heparin, Danaparoid, Hirudin and Related Compounds, and Argatroban,"Arch
Pathol Lab Med, 1998, 122(9):799-807.
Olson JD, Arkin CF, Brandt JT, et al, "College of American Pathologists
Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy.
Laboratory Monitoring of Unfractionated Heparin Therapy,"Arch
Pathol Lab Med, 1998, 122(9):782-98.