11 jgb-0099

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PROTHROMBIN TIME & INTERNATIONAL NORMALIZED RATIO
IN PEDIATRIC LIVER DISEASE
1Senior Resident, Department Of Pediatrics , VSS Medical College & Hospital, Burla, Sambalpur, Odisha 2Associate Professor, Department Of Pediatrics , VSS Medical College & Hospital, Burla, Sambalpur, Odisha 3Senior Resident, Department Of Pediatrics , VSS Medical College & Hospital, Burla, Sambalpur, Odisha Abstract
As we all know that Prothrombin Time (PT) / International Normalized Ratio(INR) is used to assess the extrinsic and common pathway of blood coagulation particularly Factor VII estimation. In the present scenario all the laboratories are doing PT /INR estimation & we all are interpreting the value in the management of patients irrespective of the nature of liver disease. The laboratories doing PT /INR estimation is for monitoring oral anticoagulant (Warfarin) therapy not for liver disease per se.We serially observed the 20 liver disease patients for last six months. Some of them (12) having PT/INR value within normal range & some of them (8) having very high value of PT/INR.These 8 patients having very high PT/INR value improves without any interventions showing the falsely high value of PT/INR.We just want to reflect that PT/INR estimation for liver disease per se & for oral anticoagulation therapy monitoring are two different thing and don't be panic at first viewing of such very high PT/INR value in liver disease patients. Key words: Prothrombin Time(PT), International Normalized Ratio(INR), International Sensitivity Index(ISI), International Sensitivity Index Liver Disease (ISI LD), Warfarin, LFT(Liver Function Test). Date of Online: 28-12-2013
INTRODUCTION
Before explaining the theme we just refreshing the steps of hemostasis1. There are three phases of
hemostasis given in fig-12
1.Vascular spasm or vasoconstriction (smooth muscle of tunica media contract)
The role of vasoconstriction is to reduce blood loss & the injured endothelia get sticky, contact each
other from opposing ends and seal off the blood vessel form both ends.
2. Platelet plug formation or platelet aggregation (Positive feed-back loop)
The role of the platelet aggregation or plug is to physically seals the broken blood vessel, actin-
myosin complex in the aggregated platelets contract to compact the plug,vasoconstrictors are released
from plug: serotonin, epinephrine, thromboxane A2, chemicals are released from plug to stimulate
coagulation.
3. Blood coagulation/clotting (Fig-2 describes the steps of blood coagulation3)
The transformation of blood from a liquid into a solid gel, formed on the top of the platelet plug,
strengthens the platelet plug. Fig-3/44 describes it elaborately.
Events after blood clotting1 1.Clot retraction: Platelets contract, squeezing serum from the clot.Serum:plasma without clotting proteins 2. Blood vessel repair: Platelet-derived growth factor stimulates rebuilding the wall of the blood vessel. 3. Fibrinolysis After repair the blood clot is removed. Done by the fibrin digesting enzyme: plasmin.Plasminogen (inactive) → → plasmin(active). The prothrombin time was discovered by Dr Armand Quick5 and colleagues in 1935, and a second method was published by Dr Paul Owren, also called the "p and p" or "prothrombin and proconvertin" method. The INR (also known as standardised prothrombin time) was introduced in the early 1960s, by Dr. Jack Hirsh6 and colleagues at McMaster University School of Medicine, when it turned out that Journal of Global Biosciences Vol. 2(6), 2013 pp. 241-246
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there was a large degree of variation between the various prothrombin time assays, a discrepancy
mainly due to problems with the purity of the thromboplastin (tissue factor) concentrate. The INR
became widely accepted worldwide, especially after endorsement by the World Health Organization.
A revised international procedure for standardization of the prothrombin time (PT) in anticoagulant
control was accepted by the international committees7 in 1941 and approved by the World Health
Organization8. International Normalized Ratio (INR), which standardizes prothrombin time (PT)
during oral anticoagulation, has been extended to standardize PT in liver diseases and is included in
prognostic models such as the Model for End stage Liver Disease (MELD)9 introduced in 1960.
INR or standardized prothrombin time, as the name suggest needs a detailed clarification of
PT(prothrombin time). The PT test is used to monitor patients taking certain medications as well as to
help diagnose clotting disorders.
A sample of the patient's blood is obtained by venipuncture. The blood is decalcified (by collecting it
into a tube with oxalate or citrate ions) to prevent the clotting process from starting before the test.
The blood cells are separated from the liquid part of blood (plasma) by centrifugation. The PT test is
performed by adding the patient's plasma to some source of Tissue Factor or factor III(e.g.: a protein,
thromboplastin, from homogenized brain tissue10) that converts prothrombin to thrombin (Fig-5). The
term thromboplastin refers to a complex mixture of tissue factor and phospholipids prepared from
tissue extracts of animal or human origin. The mixture is then kept in a warm water bath at 37°C for
one to two minutes. Calcium chloride (excess quantities of ionized calcium) is added to the mixture in
order to counteract the sodium citrate and allow clotting to start. The test is timed from the addition of
the calcium chloride until the plasma clots. This time is called the Prothrombin Time.
PT is the time taken for clotting of citrated plasma after the addition of Ca+2 & standardized
reference thromboplastins, it is expressed as ratio(PT ratio). PT ratio is PT of Pt/PT of a pool of
plasma from healthy subjects on no medication. Depending upon the variability of thromboplastins,
different results obtained in different laboratories. So to standardize PT measurement Internationally
,each thromboplastin is assigned with a ISI and Pt's PT is expressed as INR11where INR = (PT
Ratio)ISI.
ISI(International Sensitivity Index)12 is supplied by the manufacturer of the test reagent & varies upon
test reagent and instrument. It is the relative sensitivity of PT [determined from a given batch(pool of
Pt. in laboratories) of thromboplastin] to decreases in the Vit. K dependent coagulation factors in
comparison with a WHO human thromboplastin standard. The ISI is a numerical value that reflects
the responsiveness of a given thromboplastin to reduction of the vitamin K-dependent coagulation
factors13. If the INR is outside the target range, a high INR indicates a higher risk of bleeding, while a
low INR suggests a higher risk of developing a clot. Reagents with low ISI are more sensitive to
effect of Vit. K antagonist i.e.,PT is prolonged to a greater extent than to a less sensitive reagents with
high ISI14.
However, mechanisms of PT prolongation in liver diseases(DECREASE SYNTHESIS OF
FACTORII/VII/IX/X ) differ from those involved in oral anticoagulation(INHIBITION OF VIT-K
REDUCTION), and the thromboplastin reagents differ in their sensitivities to these 2
mechanisms.Some laboratories hypothesized that the use of plasmas of patients with liver disease
instead of plasmas from patients on oral anticoagulation in the calibration model for
thromboplastins15.
Calibration Procedure of Thromboplastins for the Definition of the Parameter ISI"LD". Calibration
was performed according to the WHO guidelines16 for thromboplastins and plasma used to control
oral anticoagulant therapy, with the following differences: the use of frozen instead of fresh plasmas
and the use of plasmas from patients with liver disease instead of plasmas from patients on stable oral
anticoagulation but till now no definitive ISI value for liver disease is available. Characteristics of
different Thromboplastin Reagents are given in Table-117
Time required for clotting of plasma after adding thromboplastin is called PT(N =12-13sec,>16sec =
abnormal, ≥ 20sec = definite abnormality)18.Blood collected must be used with in 2hr if at room temp
or with in 4 hr if cold & should be frozen if not used with in this period. Frozen sample should be
thawed at 370C before use.PT measures the activity of factor I, II, V, VII, X. PT dose not prolong
until the conc. of these factor is <30% or until fibrinogen <100mg/dl. Isolated PT prolongation
indicates factor VII deficiency. Normal value of ISI is 1 to 218. Normal value of INR is 0.8 to 1.2
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without anticoagulation,2 to 3 with anticoagulation, 2.5 to 3.5 with intense anticoagulation18.Table-2
demonstrates the PT values in various age groups19.
PATIENTS AND METHOD:
Twenty (20) cases were included in this study review in last six months. Inclusion criteria being
children admitted to our hospital for liver diseases not taking any anticoagulation. Detailed clinical,
hematological & LFT evaluation was done for every patients. Out of 20, 12 patients found to have
normal PT / INR (mean=12.37±0.88sec) values and 8 patients were having PT / INR values very
high(mean=20.13 ±1.78 sec) reflecting very severe liver failure. But these 8 patients though having
high PT / INR, surprisingly improves without any active interventions as the parents were extremely
poor. Thanks to the God for their survival, but the thing is to think that was their PT / INR reports
were correct, if yes then how they survived & if no then what is the fact. Calculation of mean PT /
INR was done by SPSS v 16 software.
DISCUSSION:
Out of 20 liver disease patients, 12 having PT / INR values normal & 8 patients showed high PT /
INR values.This differences most probably due to ISI values20 used by the laboratories or lessly due
to false positive21 / false negative22 results. Table-3 shows how PT / INR values differ according to
various ISI values. This table demonstrate at a constant INR value PT value decreases as ISI values
increases.Fig-6 demonstrate how different thromboplastins influence the PT ratio and INR. Table-4/5
shows various causes of false positive or negative PT / INR. As the thromboplastins are supplied by
different laboratories, the ISI values will be different but unfortunately the ISI values used by
different laboratories is for monitoring oral anticoagulant therapy & till date ISI for liver disease(ISI
LD) is not available, the PT / INR estimation we are sending is not the ideal one.
CONCLUSION:
PT ESTIMATION WITHOUT INR IS MEANINGLESS INR FOR LIVER DISEASE IS DIFFERENT FROM THAT OF INR FOR TILL DATE INR FOR LIVER DISEASES NOT AVAILABLE(UNDER RESEARCH) INR CAN BE USED FOR APTT ESTIMATION , BUT INR MEANS PT ESTIMATION ISI IS VERY IMPORTANT IN CALCULATION OF INR ISI IS DIFFERENT FOR LIVER DISEASE & ANTICOAGULATION THERAPY WE MUST CHECK THE ISI VALUE BEFORE INR ESTIMATION ACKNOWLEDGEMENT:
The authors would like to thank H.O.D.Pediatrics,V.S.S.MC and the participating clinicians for this
study review and Cyber Zone for assistant with the draft manuscript preparation.
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