MLT GlobeLaboratory Operations: Phlebotomy & specimen handling – page 2
42 Laboratory Operations MCQs on Phlebotomy & specimen handling with answers and explanations.
For a fingerstick in an adult, the recommended site is the:
The middle and ring fingers have enough tissue; the thumb has a pulse and the little finger has too little tissue over bone.
In a skin puncture, the first drop of blood is usually wiped away because it:
The first drop is diluted with tissue fluid and may contain alcohol residue, which can affect results.
Which test uses heparinized blood because EDTA and citrate would change cell volume or salt balance?
Osmotic fragility needs red cells in their natural osmotic environment, so heparin is used. Coagulation and ESR tests use citrate; the DAT uses EDTA.
Potassium oxalate prevents blood from clotting by:
Oxalate removes calcium by forming insoluble calcium oxalate. EDTA and citrate instead bind calcium in soluble complexes; heparin works through antithrombin.
Serum potassium is very high and calcium very low in a well patient. The most likely cause is:
K2EDTA adds potassium and chelates calcium. The correct order of draw (culture, citrate, serum, heparin, EDTA, fluoride) avoids this carryover.
Only a coagulation (sodium citrate) tube is ordered, and a butterfly (winged) set is used. The phlebotomist should first:
The air in the tubing would cause underfilling of the citrate tube. A discard tube (plain or citrate) fills the dead space so the 9:1 blood-to-anticoagulant ratio is kept.
Sodium fluoride in a gray-top tube preserves glucose by inhibiting which glycolytic enzyme?
Fluoride inhibits enolase, stopping glycolysis in blood cells. Hexokinase is used as a glucose assay reagent, not the target of fluoride.
A tourniquet is left on for 4 minutes while the phlebotomist searches for a vein. Which result is most likely to be falsely increased?
Prolonged tourniquet time causes hemoconcentration: water and small molecules move out of the vein, raising large molecules and protein-bound substances such as total protein and calcium. The limit is 1 minute.
A patient is asked to repeatedly clench and pump the fist during venipuncture. Which analyte is most likely to be falsely increased?
Muscle contraction releases potassium from muscle cells into local blood, falsely raising serum potassium. Fist clenching should be avoided during collection.
A grossly hemolyzed serum sample would cause the greatest false increase in which analyte?
Red cells contain far more LD than plasma, so hemolysis releases it and falsely raises LD (along with potassium and AST). Sodium and chloride are higher in plasma than in red cells.
A plasma ammonia is ordered. Which handling is correct?
Ammonia rises in vitro from red cell metabolism and deamination of amino acids. The tube (usually EDTA or heparin) should be chilled and tested quickly.
A cryoglobulin test is ordered. How should the specimen be handled until the serum is separated?
Cryoglobulins precipitate in the cold. If the sample cools before separation, they are lost in the clot and the result is falsely negative, so it is kept at 37 °C.
For a heel puncture on a newborn, CLSI recommends that the puncture depth should not exceed:
A depth of no more than 2.0 mm on the medial or lateral plantar heel avoids injury to the calcaneus, which can cause osteomyelitis.
According to CLSI guidance, serum or plasma for routine chemistry should be separated from the cells within what time after collection, unless stability data allow longer?
CLSI recommends physical separation within 2 hours to limit glycolysis and leakage of cellular contents such as potassium. Longer contact falsely lowers glucose and raises potassium.
A skin puncture collection needs an EDTA tube, a lithium heparin tube and a serum tube. According to CLSI, which should be collected first?
In skin puncture (after any blood gas sample), EDTA is collected first to limit platelet clumping and clotting, then other additive tubes, and serum last. This differs from the venous order of draw.
A blood alcohol (ethanol) level is ordered. Which skin preparation is correct?
Alcohol antiseptics could contaminate the sample and raise results, so a non-alcohol antiseptic is used. The site must still be cleaned to prevent infection.
An arterial blood gas syringe arrives with a large air bubble that was not expelled. For a patient with pO₂ of 60 mm Hg, the most likely effect is:
Room air has a pO₂ of about 150 mm Hg and almost no CO₂, so gases equilibrate: pO₂ rises toward 150 and pCO₂ falls. Bubbles must be expelled immediately after collection.
For coagulation tests, the correct ratio of blood to 3.2% sodium citrate in a light-blue tube is:
A 9:1 ratio is required; under-filling increases the relative citrate and falsely prolongs clotting times.
For the Westergren ESR method, blood is mixed with sodium citrate in a ratio of:
The Westergren ESR uses 4:1 blood to citrate (black-top tube), unlike the 9:1 ratio used for coagulation tests.
A serum lithium level is requested for a patient on lithium therapy. Which tube must be avoided?
Lithium heparin adds lithium to the sample and gives a falsely high result.