Clotting screens

The clotting screen, which includes prothrombin time (PT) and international normalised ratio (INR), is not part of the standard LFT but is essential for the assessment of a patient s liver fimction. As the liver is responsible for synthesising clotting factors this is the key marker for determining and monitoring liver function trends. 

LFTs cannot be considered accurately without the clotting screen. It is the key method of determining functional capacity of the liver. 

Remedying the problem of clot formation and its detrimental effect on sample transfer has also been addressed in a recent publication from the Johnson Johnson Pharmaceutical Research and Development Laboratories161 where workers designed a novel 96-well screen filter plate consisting of 96 stainless steel wire-mesh screen tubes (Figure 1.42). 

Receiving a transfusion of HIV-infected blood. Since transfusion involves placing foreign blood or blood products directly into the recipient s bloodstream, the necessary conditions for HIV transmission are present. After screening of the blood supply by antibody tests began in 1985, the risk was low that the blood or blood product involved in transfusion was infected, except for hemophiliacs, who require a clotting factor extracted from the blood of many different donors. 

Laboratory assessment includes indicators of general operative health (e.g., electrolytes, acid-base status, clotting profile, full blood cell count, and cross-matching). In addition, full human leukocyte antigen (HLA) tissue typing is undertaken, in addition to a full screen for infectious diseases, particularly cytomegalovirus (CMV), hepatitis, herpes, and HIV status, as these infections can be activated by immunosuppressive therapy. 

Maternal serum specimens can be obtained from nonfasting women by standard phlebotomy techniques. UE3 is the least stable of the four analytes currently used for screening, and consequently, requirements for collection, storage, and shipment are dictated by this analyte. The uEj concentration increases in blood at room temperature and at 4 °C, because the conjugated forms can spontaneously deconjugate to form the parent hormone.Therefore collected blood should be allowed to clot and then serum should be removed promptly. If serum separator tubes are used, specimens should be centrifuged promptly after collection. Shipment of whole blood is not preferred. If whole blood is shipped through the mail, next day delivery is essential. UE3 is stable in serum for up to 7 days at 2 C to 4 C (unpublished data, Foundation for Blood Research, Woman and Infant s Hospital, and ARUP Laboratories). The concentration of UE3 increases when sera have been stored for more than 4 days at room temperature. 

The prothrombin time (PT), as proposed by Quick, is the most commonly performed coagulation function test. It is used to monitor oral anticoagulant therapy and also as a preoperative screening test to warn of possible bleeding risk in patients with a personal or family history of bleeding (Table 36-2). Measured clotting times are extremely dependent on the animal and tissue source and the quality of the thromboplastin used. Variability can be expected because of the assay s dependence on the number of tissue factor molecules and the quantity of... 

In general, the Factor VII activity estimated by a substrate assay correlated well with Factor VII activity determined by a clot-endpoint assay (P4, S13). However, some differences have been seen. Activation of Factor VII by kaolin or by exposure to cold temperatures (2-8°C) increased the level of clotting Factor VII by 4-7 times but bad no effect on the amount detected by the substrate assay (S9, TIO). It has been postulated that the partially carbox-ylated molecules of Factor VII present in patients plasma during anticoagulant therapy are detected by a substrate assay but not in a clot-endpoint test (P4). These differences between the two assay systems have been used to screen for hereditary Factor VII deficients and to detect the presence of Factor Vila in thrombotic disorders and in components for transfusion therapy (S13). 

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