11.3 Coagulation: PT, PTT, INR, Heparin, and Warfarin

CMLA III.6.C covers coagulation and hemostasis: know heparin, warfarin, protime, PT, and PTT; perform the prothrombin time (PT) and know which medication it monitors; perform the partial thromboplastin time (PTT) and know which medication it monitors; and understand the linear range for the international normalized ratio (INR). Collection details sit on the same competency because a wrong fill ruins both assays. A Certified Medical Laboratory Assistant (CMLA) may perform point-of-care (POC) PT/INR and assigned coagulation methods that are waived or otherwise within training, and must collect a valid light-blue tube for methods run by qualified personnel. Independent high-complexity special coagulation panels are not assistant sign-out work.

Quick Answer: PT (protime) monitors warfarin (Coumadin). INR is a standardized PT ratio. PTT monitors unfractionated heparin; low-molecular-weight heparin (LMWH) is often followed with anti-Xa, not treated as a PTT clone. Light-blue citrate is 9:1 blood-to-anticoagulant—no short draws. Report INR only inside the method's linear range; therapeutic targets come from the order (many atrial-fibrillation/VTE protocols use about 2–3; mechanical valves may be higher).

Hemostasis in one paragraph so the tests make sense

Primary hemostasis is the platelet plug (Chapter 11.1). Secondary hemostasis is the coagulation cascade that turns fibrinogen into fibrin. Laboratory PT and PTT do not watch the patient bleed; they time how long citrated plasma (or a whole-blood POC cartridge) takes to clot after a reagent adds back calcium and a pathway-specific activator. The CMLA does not memorize every numbered factor. You do need the medication pairing and the tube.

PT, protime, INR, and warfarin

Prothrombin time (PT), also called protime on older orders, is the clotting time of the extrinsic and common pathways (tissue-factor/factor VII leading into X–V–II–fibrinogen). The reagent is thromboplastin plus calcium.

Warfarin (brand name Coumadin and others) is an oral vitamin K antagonist. It reduces functional factors II, VII, IX, and X (and proteins C and S). Factor VII is short-lived, so PT/INR is the monitoring test for warfarin. PT does not monitor unfractionated heparin as its primary assay, does not measure platelet aggregation from aspirin, and is not an INR you invent without a PT.

Different thromboplastin reagents have different sensitivities. The INR was created so warfarin results can be compared across laboratories:

INR = (patient PT ÷ mean normal PT) raised to the ISI

ISI is the International Sensitivity Index assigned to that reagent/instrument combination. You do not calculate ISI at the chair. The device or the core-lab analyzer applies it after a valid PT. Teach INR as a standardized PT ratio, not as a different blood test drawn in a different tube.

Linear (analytic/reportable) range for INR. Every PT/INR method is valid only between the lowest and highest INR (or PT) the manufacturer calibrated and claims. If the device prints an INR above that range, you do not extrapolate, guess, or report a clinic-habit number. Follow the IFU: often a repeat, a dilution performed only if the method is validated for it (usually not a waived-meter CMLA task), or report as greater-than the top of the range and notify per SOP. Below the range, the same rule: do not invent 0.5 because the cartridge error-coded. Linearity here is the same idea as Chapter 4: results outside the proportional range are not reported as-is.

Therapeutic INR targets are ordered by clinicians. AMT does not publish a single universal therapeutic INR as an examination fact. Many atrial-fibrillation and venous-thromboembolism protocols use a target about 2–3. Some mechanical-valve protocols use a higher target. Always use the order and the laboratory/clinic SOP. Do not tell a patient their INR is fine because it is 2.5 if the ordered range is different, and do not fail a CMLA item because you memorized only 2–3 as if it were the only legal range.

POC INR meters (often waived) use a drop of capillary or venous whole blood on a coded strip. Insert the lot-specific code chip or calibration, run required liquid QC, and test only the specimen type the IFU lists. Using the previous lot's chip, an unapproved specimen, or extra incubation is off-label (Chapter 5).

PTT and heparin

Partial thromboplastin time (PTT), commonly activated PTT (aPTT) in current methods, times the intrinsic and common pathways (XII, XI, IX, VIII, then the common pathway). The reagent is a platelet-phospholipid substitute plus an activator, then calcium.

Unfractionated heparin (UFH) potentiates antithrombin and prolongs aPTT in a dose-responsive way on most protocols. Perform PTT and know that it monitors unfractionated heparin. Heparin in a green-top chemistry tube is the additive, not the patient's infusion; you cannot use a heparin-plasma chemistry tube as a PTT specimen.

Do not oversimplify LMWH. Low-molecular-weight heparin (for example enoxaparin) often produces a smaller, less predictable aPTT change. Many protocols monitor LMWH with an anti-factor Xa assay, not with PTT, and some patients are not monitored with a lab test at all. If a stem says LMWH, do not automatically answer PTT as if it were UFH. If a stem says unfractionated heparin, PTT (aPTT) is the CMLA pairing. Direct oral anticoagulants and anti-Xa details beyond that pairing are outside the AMT term list; do not invent them as the exam answer.

Warfarin can prolong PTT somewhat; heparin can prolong PT somewhat. Those cross-effects do not change the monitoring pairing: PT/INR → warfarin; PTT → unfractionated heparin.

Light-blue 9:1, short draws, mixing, and discard tubes

Coagulation specimens are light-blue sodium citrate (laboratory methods typically 3.2% citrate). The fill is 9 parts blood to 1 part anticoagulant. That ratio is why the fill line is not decorative. Chapter 7 already rejected short-draw citrate for order-of-draw reasons; here it ruins the assay you are about to perform.

• Underfill (short draw): too much citrate for the amount of blood. Excess citrate then binds the calcium the reagent adds, and PT/PTT falsely prolong. Reject and recollect. Do not add saline to the fill line. Do not pour EDTA or heparin blood into the blue top.
• Overfill: too little citrate relative to blood; clotting may begin in the tube and times are invalid. Reject.
• Mix by gentle inversion immediately (typical IFU about 3–4 inversions). Shaking hemolyzes cells and can activate platelets.
• Clots in a citrate tube: reject. The factors you are measuring have already been consumed.

Butterfly (winged) sets contain air in the tubing. That air occupies volume in the first tube and short-draws citrate. When the SOP requires it, collect a discard (clearing) tube first so the citrate tube fills 9:1. A non-additive discard or another citrate tube used as discard—follow the SOP; do not use a clot-activator or EDTA tube as the tube that will be tested.

Line draws: heparin flushes and dead space contaminate coagulation tests (falsely long PTT, sometimes PT). Follow the SOP: waste/discard volume from the line, then fill citrate. If the SOP says not to use that line for coagulation, recollect peripherally. Never draw a PTT from a running heparin line and call it the patient's result.

Many laboratory PT/PTT methods need platelet-poor plasma after centrifugation. Assistants who process tubes follow the SOP for spin time, speed, and not delaying plasma on cells. POC INR meters that use whole blood skip that spin—only if the IFU says whole blood.

Scenario

A provider orders PT/INR for warfarin and aPTT for a patient on an unfractionated heparin drip. You fill a light-blue tube to the line after a butterfly discard tube, invert it 3–4 times, and never pour from the lavender CBC tube when the blue top underfills. The POC INR meter reads above its reportable range. You do not write 2.5 because many atrial-fibrillation protocols use 2–3. You follow the IFU (repeat or report greater-than and notify). For the heparin drip, you send a properly filled citrate tube for aPTT, not a chemistry heparin tube and not an LMWH anti-Xa assumption.

Exam traps

• Swapping the medications: PT is not the UFH assay; PTT is not the warfarin assay.
• Treating LMWH as if PTT always replaced anti-Xa.
• Calling INR a different draw than PT.
• Inventing one AMT-wide therapeutic INR and reporting values outside the linear range.
• Short-draw citrate salvaged with saline, or a butterfly/line specimen without the SOP discard.

When CMLA shows PT, PTT, or INR, pair the test with the drug, name the 9:1 blue top, and keep the number inside the method's range and the ordered target.