Introduction
Point-of-care testing is a rapidly expanding area of biomedical science and is now essential to efficient patient care. As the name implies, the goal is to collect the specimen to be tested and obtain the results in a short period of time ( Fig. 14.1 ). This ideally should be done at, or at least near, the location of the patient.
Historically, blood gas analysis was the forerunner of point-of-care testing, and blood glucose analysis revolutionized diabetic care also.
Now tests as diverse as human immunodeficiency virus (HIV), C-reactive protein (CRP), haemoglobin A1C (HbA1C) and cardiac markers can be rapidly obtained at the point of care. This chapter will consider only a few of the available point-of-care tests and will go into detail on some of the more common machines seen in healthcare theatres.
HemoCue
A reliable compact device that measures haemoglobin concentration.
Components
- 1.
A handheld and portable device houses a photometer.
- 2.
It has a disposable microcuvette into which a drop of capillary, venous or arterial blood is placed.
Mechanism of action
- 1.
The microcuvette contains a dried reagent mixture. Sodium deoxycholate haemolyses red blood cells, releasing the haemoglobin. Sodium nitrite converts haemoglobin to methaemoglobin which together with sodium azide, gives azidemethaemoglobin.
- 2.
A specific photometer is housed in the device and measures the absorbance at 570 nm and 880 nm.
- 3.
Results are displayed digitally almost immediately.
Problems in practice and safety features
- 1.
Quality control is necessary, to ensure accuracy is maintained.
- 2.
The microcuvettes need to be stored in a dry environment.
Hemochron Junior
This is a compact, mains or battery powered device that measures activated clotting time (ACT) ( Fig. 14.2 ). The ACT is used to monitor unfractionated heparin therapy.
Each different version of an ACT machine has its own baseline reference value. The normal range for the Hemochron Junior is usually between 90–140 seconds.
After a dose of unfractionated heparin, the measured ACT value should rise. The two versions of cuvette commonly used are low range and high range.
Components
- 1.
A handheld device contains the test chamber.
- 2.
A ‘Keypad’ and ‘Results’ display panel.
- 3.
The test cuvette contains the following activation reagents:
- a.
low range: Celite, potato dextrin, stabilizers and buffers
- b.
high range: Silica, kaolin, phospholipid, stabilizers and buffers.
- a.
Mechanism of action
- 1.
The cuvette is warmed to 37°C.
- 2.
A single drop of blood is needed to be added to the warmed cuvette.
- 3.
The result is available in minutes.
Problems in practice and safety features
- 1.
Memory function to store results.
- 2.
Printer option available.
- 3.
Quality control testing is necessary at regular intervals.
- 4.
The ACT reading is prolonged by heparin, hypothermia, haemodilution, low platetlet count, warfarin, clotting factor deficiencies, hypofibrinogenaemia, qualitative platelet abnormalities, aprotinin.
Original TEG/ROTEM
Background
Speed to clot formation and the firmness (quality) of the clot formed at the point of care can be assessed by two similar technologies. They differ in essence via the simple principles of testing the firmness of a clot as it forms. One spins a cup around a fixed pin (thromboelastography [TEG]). The other spins a pin in a fixed cup (rotational thromboelastometry [ROTEM]).
Prothrombin time (PT), activated partial thromboplastin time (APTT) and thrombin time (TT) are traditional coagulation laboratory-based tests, with the associated time delay to results being available.
Because of the speed of access to results with TEG/ROTEM, these point-of-care machines are rapidly becoming the gold standard for the treatment of bleeding in major trauma, cardiac and liver transplantation surgery and massive haemorrhage resulting from other causes.
A small volume of whole blood is added to a cup containing a clot activator assay. Other cups that contain heparinase (an enzyme that neutralizes heparin) with the clot activator, and other reagents are also available.
How it works
The cup is placed on the machine, and a sensor pin is inserted into the blood. Rotational movement occurs between the pin and the cup in one direction, followed by a short pause. Rotation then occurs in the opposite direction. This back and forth rotation continues throughout the analysis, and is intended to represent sluggish venous blood flow. The whole system is kept at 37°C. As the clot begins to form, the blood becomes more viscous and exerts more resistance to rotation. This resistance is detected and plotted on a graph of clot strength against time.
TEG 6s
This is the most modern and truly portable point-of-care device at the time of publication ( Fig. 14.3 ). This device uses a resonance method instead of a cup and pin (as discussed previously) to produce the clot formation measurements.
