Unit 5: Specimen Handling and Processing

• Topic 01

  Tube Mixing Requirements by Additive Type

  15 questions
• Topic 02

  Centrifugation — Speed, Time, and Serum vs Plasma

  15 questions
• Topic 03

  Haemolysis, Lipaemia, and Icterus — Causes and Impact on Results

  15 questions
• Topic 04

  Specimen Labelling — Required Elements and Barcoding

  15 questions
• Topic 05

  Transport Conditions — Temperature, Light Sensitivity, and Time Limits

  15 questions
• Topic 06

  Rejected Specimens — Common Causes and Corrective Action

  15 questions
• Topic 07

  Chain of Custody — Forensic and Medico-Legal Specimens

  15 questions

A few questions from this unit, with the answer and a full explanation. The complete bank is available when you start practising.

1. How long must an SST tube be allowed to clot before centrifugation?

   • A
     2 hours at room temperature
   • B
     60 minutes in a refrigerator
   • C
     10 minutes at room temperature
   • D
     A minimum of 30 minutes at room temperature
     Correct answer
   Explanation

   The SST (serum separator tube) contains a clot activator (silica particles) that promotes clot formation, but the clot must be complete before centrifugation. CLSI guidelines specify a minimum of 30 minutes at room temperature. If centrifuged too early, the clot is incomplete and fibrin strands migrate above the gel, contaminating the serum and potentially clogging analyzer probes. Key principle: SST tubes require at least 30 minutes of clot time at room temperature before centrifugation.

2. What is icterus, and what causes the yellow discoloration of serum?

   • A
     Icterus is a turbid white appearance caused by elevated triglycerides in the circulation
   • B
     Icterus is a pink tint in serum caused by elevated haemoglobin from red cell lysis
   • C
     Icterus is yellow discoloration of serum caused by elevated bilirubin from liver disease, haemolysis, or biliary obstruction
     Correct answer
   • D
     Icterus is a dark brown color in serum caused by elevated myoglobin from muscle breakdown
   Explanation

   Icterus (from the Greek word for jaundice) describes the yellow to dark yellow discoloration of serum or plasma caused by elevated bilirubin. Bilirubin is the yellow breakdown product of hemoglobin. It accumulates in blood when liver disease impairs its processing (hepatic icterus), when biliary obstruction prevents its excretion (obstructive icterus), or when excessive red blood cell destruction releases more bilirubin than the liver can clear (hemolytic icterus). Elevated bilirubin absorbs light at wavelengths used in many photometric laboratory tests, causing interference with multiple analytes. Key principle: icterus is yellow serum discoloration from elevated bilirubin, caused by liver disease, haemolysis, or bile duct obstruction.

3. What happens if an SST tube is centrifuged before the clot has fully formed?

   • A
     Fibrin strands form above the gel, contaminating the serum and potentially clogging analyzer probes
     Correct answer
   • B
     The serum becomes hemolyzed due to disruption of red blood cells
   • C
     The tube pressure equalises causing a vacuum failure and short draw
   • D
     The clot activator is inactivated, preventing any future clot formation in the tube
   Explanation

   When an SST is centrifuged before the clot is complete, the centrifugal force disrupts the partially formed clot. Fibrin strands are pushed upward above the thixotropic gel rather than being trapped below it with the cell pellet. These fibrin threads appear as white stringy material in the serum layer and can clog analyzer probe tips, causing instrument errors and invalid results. CLSI GP44-A4 specifies a 30-minute minimum clot time to prevent this outcome. Key principle: premature centrifugation of an SST causes fibrin contamination of the serum above the gel.