Pre-analytical factors that can affect
coag test results
•
•
•
•
•
•
Underfilled tube
High hematocrit
Hemolysis
Traumatic blood draw (tissue factor)
Delay in testing
Excessive agitation of blood in tube
(platelet tests)
Effect of high hematocrit on coag tests
Elevated citrate concentration may prolong clotting times
The Prothrombin Time
Thromboplastin:
Tissue factor
Phospholipid
Calcium
Also contains a heparinneutralizing agent
Add thromboplastin (excess of tissue factor
+ phospholipid + calcium) to citrated plasma.
Not sensitive to XI, IX, VIII levels
More sensitive than aPTT to warfarin effect
Usually expressed as International
Normalized Ratio (INR)
Why the INR?
• Tissue factor in the thromboplastin may be
recombinant, or derived from human or
animal tissue
• Phospholipid composition varies among
thromboplastins
• As a result different thromboplastins have
varying sensitivity to the effect of warfarin
• The INR system makes PT results from
different laboratories comparable to one
another in patients receiving vitamin K
antagonists (not in liver disease or other
coagulopathies)
Patient
PT
INR =
Mean Normal PT
(
ISI
)
ISI (International Sensitivity Index) is reagent- and method-specific;
higher number indicates lower sensitivity to changes in clotting
factor levels
Reagent A: ISI = 1.24, mean normal = 12.6 sec
PT = 22 sec
22.0
INR =
12.6
(
1.24
)
= 2.0
Reagent B: ISI = 2.46, mean normal = 12.2 sec
PT = 16.2 sec
16.2
INR =
12.2
(
2.46
)
= 2.0
INR values with two different reagents
Patients on warfarin
REAGENT E (ISI 2.98)
REAGENT B (ISI 0.96)
PATIENT #
INR
INR
1
3.4
2.7
2
2.8
2.5
3
3.5
2.3
4
2.6
2
5
2.2
1.2
6
2.3
2.4
7
1.9
1.7
8
3
2.8
9
2.2
2.7
10
4
4
Uses of the PT/INR
• Best single test of the integrity of the fibrin
clotting system
• Detects most clinically significant acquired
coagulopathies
• Does not detect the most common
inherited clotting factor deficiencies (VIII,
IX, XI)
• Routinely used to monitor warfarin therapy
• Insensitive to heparin at usual therapeutic
concentrations
Activated partial thromboplastin
time (aPTT)
“Partial thromboplastin”
Phospholipid +
Activator (provides surface
for generation of XIIa)
Incubate citrated plasma with phospholipid +
activator (generates XIIa→XIa→IXa). Then
add calcium to allow clotting to proceed to
completion.
Not sensitive to VII level.
More sensitive to heparin than PT
Uses of the PTT
• Screen for inherited clotting factor
deficiency (hemophilia, factor XI)
• Monitor heparin therapy
• Screen for acquired coagulation inhibitors
– Factor VIII antibody
– Lupus anticoagulant
• A long PTT does not always indicate a
bleeding disorder
– Factor XII deficiency
– Lupus anticoagulant
The aPTT should be ordered selectively
Results of 1025 consecutive tests, excluding
heparin monitoring
# abnormal: 143 (14%)
Abnormal result
On anticoagulant
Liver disease
No cause found, no
bleeding
Normal on repeat testing
Known hemophilia
History of intestinal bypass
Other malabsorption (CF)
Technical problem with test
Newly dx'd bleeding
disorder
# TESTS
143
64
41
15
9
5
5
2
# PATIENTS
97
37
27
14
9
4
4
1
1
1
0
0
Robbins and Rose, Ann Intern Med 1979;90:796
What causes a long PT/INR and a normal
PTT?
• Factor VII deficiency
• Mild deficiency of
“common pathway”
factors
– Warfarin
– Vitamin K deficiency
– Liver disease
PTT PT/INR
What would cause a long PTT with a normal
INR?
• Deficiency of VIII, IX, XI
• Deficiency of a contact
factor (usually XII)
(does not cause
bleeding)
• Heparin
• Factor VIII inhibitor
• Lupus-type inhibitor
(antiphospholipid
antibody)
PTT PT/INR
What if both PT/INR and PTT are long?
•
•
•
•
•
•
Liver disease
Vitamin K deficiency
Warfarin
DIC
High level of heparin
Other inhibitor affecting
common pathway (eg,
direct thrombin inhibitor)
• Isolated deficiency of X,
V, II, fibrinogen (rare)
PTT PT/INR
Other tests
aPTT
Thrombin time
PT/INR
Thrombin time: thrombin + plasma. Very sensitive to heparin
and other thrombin inhibitors. Prolonged by low fibrinogen,
dysfibrinogenemia, high levels of fibrin degradation products.
Urea solubility: clot immersed in concentrated urea (breaks
noncovalent bonds) clot dissolves unless crosslinked by
factor XIIIa). For diagnosis of severe factor XIII deficiency (v.
rare)
Mixing Study
• Purpose: to determine whether long aPTT or PT is due
to clotting factor deficiency or circulating inhibitor (eg,
factor VIII inhibitor, heparin, lupus-type inhibitor)
• Mix patient plasma 1:1 with normal plasma, measure
aPTT or PT
• Incubate mixture for one hour, repeat aPTT or PT
– Certain inhibitors (eg, factor VIII antibody) take time to work
• Failure to correct prolonged clotting time by mixing with
normal plasma implies presence of a circulating
inhibitor
• Note: warfarin does not act as a circulating inhibitor. It
inhibits synthesis of clotting factors by the liver
Clotting factor assay
• Serial dilutions of patient plasma in
factor-deficient plasma
• Serial dilutions of normal plasma in
factor-deficient plasma (calibration
curve)
• Measure aPTTs of both sets
• Semi-log plot - % of normal factor vs
aPTT
3%
<1%
100/.5 = 200%
Lupus inhibitor or other non-competitive
clotting inhibitor → non-parallel plot
100%
% test plasma
≥50%
50%
10%
5%
1%
20
40
60
aPTT (sec)
80
100%
% test plasma
50%
Factor VIII
Inhibitor
10%
5%
1%
20
40
aPTT (sec)
60
80
Bethesda Assay for Inhibitors
• Serial dilutions of patient plasma in normal
plasma
• Incubate 2 hours
• Assay residual factor activity
• 1 Bethesda Unit neutralizes 50% of factor in
an equivalent volume of normal plasma
• Example: 1:100 dilution of patient plasma +
normal plasma → 50% residual factor
activity, so inhibitor titer is 100 BU
50%
Residual factor activity
Bethesda Assay
100 BU
1:1
1:10 1:100 1:1000
dilution pt plasma
The decline and fall of the bleeding time
• Advantage: an in vivo test that theoretically
measures both vascular and platelet function
• Disadvantages
– Poor standardization
– Accuracy depends on experience of operator
– Poor sensitivity, very poor specificity
– Does not predict bleeding risk
The bleeding time accurately detects
aspirin use
Rodgers and Levin, Semin Thromb Hemost 1990; 16:1
The bleeding time does not predict
surgical bleeding
Rodgers and Levin, Semin Thromb Hemost 1990; 16:1
Platelet function analysis
• Whole blood passed through
capillary tube coated with
collagen plus either ADP or
epinephrine (high shear)
• Time to occlusion measured
• Moderate sensitivity to
platelet function defects,
VWD
PFA-100
Bleeding time vs PFA for detection of VWD
C-ADP
C-Epi
BT
Thromb Haemost 2003;90:483
Platelet function analysis
• Advantages vs bleeding time
– In vitro test
– Well-standardized
– Somewhat better sensitivity and specificity
• Disadvantages
– Does not assess vascular function
– Does not predict bleeding risk
PFA-100
• Abnormal test result → test for specific
defects in primary hemostasis
• Test not useful if platelets <100K or if
patient taking ASA, etc
Platelet aggregometry
• Various platelet agonists added to whole
blood
– Thrombin, ADP, collagen (2 concentrations),
arachidonic acid, ristocetin (2 concentrations)
•
•
•
•
•
Aggregation decreases electrical conductance
Release measured by chemiluminesence
Significantly more sensitive than PFA
Many abnormal results nonspecific
Expensive
Saline agg
AA agg
ADP agg
Thrombin rel
Collagen agg
Low
High
2 nM ATP
Ch 1
Ch 2
AA rel
ADP rel
Risto low agg
Risto high agg
High
Low
Collagen rel
Pt
Risto low
Control
Type I VWD
Risto high
Pt
Collagen agg
Low
High
AA agg
ADP agg
Normal
High
Low
Collagen rel
AA rel
ADP rel
Took Excedrin 5 days ago
Pt
AA agg
ADP agg
Normal
Taking ASA 81 mg/d and Plavix 75 mg/d
AA rel
ADP rel
Coll agg
Low
High
Coll release
Low High
AA agg
Coll agg
PFA: Coll/ADP 91 (nl 65-120)
Coll/Epi 139 (nl 85-175)
AA rel Coll rel
Assessment of the fibrinolytic system
• Fibrinogen (dilute thrombin time assay)
• D-dimer (immunoassay)
• α2-antiplasmin activity (chromogenic
substrate assay)
• Thromboelastography
Global assessment of clotting:
thromboelastography
• Measures mechanical strength of clot vs time
• Sensitive to most major defects in fibrin clot formation, platelet
plug formation, excessive fibrinolysis
• Can also detect hypercoagulability
• Useful “point of care” test in OR, etc to guide blood product use
30 min
World J Transplant 2012;2:1
Effect of Coagulation Factor
Deficiency on TEG
Normal
Factor deficiency
Effect of platelet abnormality on TEG
Normal
Thrombocytopenia or
dysfunctional platelets
Effect of hyperfibrinolysis on TEG
Normal
Hyperfibrinolysis
Chromogenic substrate-based assay
• Peptide containing target sequence of enzyme linked to chromophore
• Colored cleavage product (in this case nitroaniline) detectable by
spectrophotometry
• Enzyme specificity determined by target sequence
• Rate of color generation proportional to enzyme activity
Examples of chromogenic assays
• Anti-Xa assay
– Patient plasma added to mixture of Xa and
chromogenic substrate (± antithrombin)
– Residual Xa activity inversely proportional
to inhibitor level
• Protein C activity
– Patient plasma + venom enzyme that
selectively activates protein C; activated
protein C cleaves substrate
Von Willebrand factor measurements
• VWF antigen: via ELISA
• VWF activity (must be sensitive to total
VWF protein, integrity of binding sites,
presence of large multimers)
– Ristocetin cofactor assay: patient plasma +
ristocetin + formalin fixed platelets
– Ristocetin unfolds VWF, which then clumps
platelets
– Alternative assay uses beads coated with
monoclonal Ab against GP1b binding site in
VWF rather than platelets
• Multimer analysis: via gel electrophoresis