Bleeding time measurement is a useful, but subjective and variable, in vivo test to evaluate hemostasis. There are several different bleeding times, including the buccal mucosal bleeding time (BMBT, a still used test for primary hemostasis in small animals), lip bleeding time (a still used test for primary hemostasis in large animals) and the cuticle bleeding time (a crude, outdated test of secondary hemostasis). A tail bleeding time is also used as a crude test of primary and secondary hemostasis in rodent models. Only the BMBT will be discussed here.
Specific lancets are need to perform a BMBT. These produce a standardized cut in the mucosa, the depth of which are sufficient to provoke (and thus evaluate) platelet plug formation but are not deep enough to necessitate fibrin formation, making the test specific for primary hemostasis (as long as it is performed properly). There are two spring-loaded lancet devices, the Surgicutt® or Simplate II®. Surgicutt devices also come with lancets for adults and children (including neonates). These lancets differ in the length and depth of the incision. The Simplate II produces two incisions that are 6 mm long x 1 mm deep. The Surgicutt Adult and Junior produce single cuts that are 5 or 3.5 mm long, respectively, and 1 mm deep, whereas Surgicutt Newborn produces a single cut that is 2.5 mm long and 0.5 mm deep.
To perform the BMBT, the upper lip is folded up and secured with a gauze strip tied around the maxilla or around both the maxilla and mandible. A small incision is made in the mucosa above the premolars using the desired device. Areas with visibly engorged vessels should be avoided (the gauze strip is likely too tight if vessel engorgement is obvious). Blood that wells up from the incision is blotted with filter paper applied near (but not touching) the incision. A stopwatch is started when the incision is made and stopped when a crescent of blood no longer develops on the filter paper. Ideally, the same person should perform the BMBT to minimize variability (which is still very high)
The BMBT is mostly used as a screening tool for primary hemostasis. It is usually not done as a first line test for any particular disorder (it is insensitive and not specific for any particular disorder of primary hemostasis), but is only performed in animals with bleeding attributable to a primary hemostatic defect (usually mucosal bleeding) but with normal platelet counts, coagulation screening tests (PT, APTT, TCT) and vWf:Ag concentrations, i.e. it is used to detect a possible thrombopathia (platelet function defect). However, due to the variability of the test, results should be interpreted with caution and should be verified by more specific testing. Platelet function analyzer testing is preferable to the BMBT, but the latter is often the only available test for general practitioners. The BMBT is also not predictive of surgical hemorrhage, therefore its use for this purpose is not recommended. Only prolonged BMBT are clinically relevant.
Reference intervals for BMBT have been established for dogs and cats but are device specific (Aumann et al., 2013).
- Unanesthetized and anesthetized dogs of mixed breed with the Simplate II: BMBT can be as long as 3.2 minutes and 4 minutes, respectively; i.e. > 4 minutes is prolonged. Anesthesia and sedation can mildly prolong the BMBT; the BMBT can be readily performed on unsedated or unanesthetized dogs, which would reduce any potential impact these drugs have on the bleeding times.
- Unanesthetized greyhounds with the Surgicutt Adult: 53-235 seconds or < 4 minutes (Sato et al., 2000).
- Anesthetized cats (valium, dexmedetomodine and morphine) with Surgicutt Adult: 34-105 seconds or <2 minutes (Alatsaz et al., 2014).
The test is crude and quite subjective, and has poor repeatability (high intra-observer and inter-observer variability). Indeed, the BMBT can vary by as much as 80 to 87 seconds in an individual dog and cat, even when performed by a single observer (Sato et al 2000, Alatsaz et al 2014). The low reproducibility is likely due to the following technical variables:
- Depth of cut: Although the lancet is designed to produce a shallow cut, if pressure is placed on the device as it sits on the buccal mucosa, the cut will be deeper and the test may trigger secondary hemostasis (and will no longer test only primary hemostasis).
- Tightness of gauze tie: Vascular stasis or engorgement caused by a tight gauze tie will influence the time, potentially causing more bleeding and longer times.
- Filter paper technique: The filter paper should not touch the cut, since this may interfere with platelet plug formation and prolong the test.
The BMBT tests primary hemostasis, i.e. platelets (number and function), vessel wall defects (which are quite rare) and vWf (amount and function). When done properly, a BMBT is normal in disorders of secondary hemostasis, including hemophilia A or B, and anticoagulant rodenticide toxicosis.
Prolonged BMBT could reflect any of the following disorders:
- Thrombocytopenia: The BMBT may be prolonged in dogs with (< 90,000 platelets/μL).
- vWD: The BMBT will be prolonged in dogs with moderate to severe deficiencies in vWf (typically < 20% vWf:Ag concentrations). Dogs with type II vWD, which have abnormal vWf structure as well as amount, may have a long BMBT at higher vWf:Ag concentrations. The BMBT may be very prolonged (>10 minutes, to infinite) in dogs with severe vWD (type III). Since dogs with vWf:Ag concentrations higher than 20% matt not have prolonged BMBT, the test is not recommended for use as a screening tool for this inherited defect and vWf:Ag concentrations should be measured instead.
- Thrombopathia: Platelet function defects can result in thrombopathia and prolong the BMBT. These defects can be inherited (e.g. Chediak-Higashi syndrome in cats) or acquired diseases, e.g. uremia and anti-platelet therapy. However, the BMBT is insensitive to these defects and studies have shown that the BMBT is only mildly prolonged (within intra-observer variability) after treatment of dogs with aspirin (Jergens et al., 1987).
- Anemia: No definitive studies have been done in dogs, but anemia in human patients will prolong the BMBT. It is possible that the prolonged BMBT reported in dogs with uremia is secondary to anemia versus truly reflecting a uremic-induced thrombopathia.
Further information on the BMBT is provided by the Comparative Coagulation Laboratory of the AHDC.