🔬 Coagulation Screen

A set of laboratory tests (PT, APTT, Fibrinogen) used to evaluate the blood's ability to clot via the intrinsic, extrinsic, and common pathways.

Indications include the investigation of easy bruising, epistaxis, or excessive bleeding after minor trauma/surgery. It is performed as part of a preoperative screen for major surgery and in the management of acute haemorrhage or sepsis (to screen for DIC). It is also used to monitor anticoagulation therapy and to assess synthetic function in patients with suspected or known liver disease.

Venous blood is collected into a light blue-top tube containing sodium citrate, which acts as a reversible anticoagulant by chelating calcium. It is vital that the tube is filled to the marked line to maintain the correct 9:1 blood-to-citrate ratio; underfilling leads to falsely prolonged results. In the lab, the sample is centrifuged to obtain plasma, and reagents are added to trigger the extrinsic (PT) and intrinsic (APTT) pathways, measuring the time taken for a fibrin clot to form.

Normal findings reflect Prothrombin Time (PT), Activated Partial Thromboplastin Time (APTT), and Fibrinogen levels within the laboratory's established reference ranges. A normal INR (International Normalised Ratio) is approximately 1.0 in a patient not taking anticoagulants. These results suggest that the secondary haemostasis pathways are functioning adequately to form a stable clot.

Interpretation involves identifying which part of the coagulation cascade is affected. An isolated prolonged PT usually points to factor VII deficiency or early Vitamin K antagonism (warfarin). An isolated prolonged APTT suggests factors VIII, IX, or XI deficiency. If an abnormal result is found, a 'mixing study' may be performed: if the time corrects with normal plasma, a factor deficiency is likely; if it doesn't, an inhibitor (like Lupus Anticoagulant) is suspected.

A prolonged PT suggests Vitamin K deficiency, liver disease, or warfarin use. A prolonged APTT indicates deficiencies in the intrinsic pathway (e.g., Hemophilia A/B) or the presence of heparin. If both PT and APTT are prolonged, this suggests a common pathway defect, severe liver failure, or Disseminated Intravascular Coagulation (DIC). Conversely, a low fibrinogen is seen in massive haemorrhage or DIC, indicating a high risk of catastrophic bleeding.

The screen is essential for the preoperative assessment of surgical patients to predict bleeding risk. It is used to monitor the efficacy of certain anticoagulants (PT for warfarin, though usually reported as INR) and to guide the administration of blood products (like Fresh Frozen Plasma or Cryoprecipitate) in major trauma or haemorrhage. It is also a key marker of liver synthetic function, as most clotting factors are produced by hepatocytes.

The most common pitfall is 'clotted' or 'underfilled' samples, which yield inaccurate results and must be rejected by the lab. Clinicians often mistakenly order a coagulation screen for patients on Direct Oral Anticoagulants (DOACs); while these drugs may slightly prolong PT or APTT, the standard screen cannot be used to accurately monitor their therapeutic effect or levels.

The standard screen does not measure platelet function or factor XIII activity, so a patient can have a normal screen but still have a clinical bleeding disorder (e.g., Von Willebrand Disease or aspirin effect). These tests are 'static' and performed in a test tube, which may not perfectly reflect the complex 'cell-based model' of coagulation occurring within the human body.

Understand the difference between PT and APTT and which factors they represent. Remember that the INR is a standardised version of PT specifically for warfarin monitoring. In DIC, you expect prolonged PT/APTT, low fibrinogen, and low platelets.