Frequently asked questions about the CoaguChek systems

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1. What are the advantages of Self-Monitoring of the INR for the patient?

The advantages of Self-Monitoring at a glance:

  • Helps the patient to contribute to the treatment - leading to better compliance
  • More frequent testing means longer time in the therapeutic range (thus fewer complications)
  • Fewer trips to the hospital lab for testing
  • Greater quality of life/independence at work, on holiday, at home
  • Better and safer fine-tuning of the treatment compared to usual testing
  • No venous punctures (only a drop of blood from the fingertip)

2. How often should measurements be carried out?

The measurement interval is usually recommended at the time of the training session and the patient should discuss it with the doctor. Roche and the ISMAA recommend weekly measurement, but it may vary from country to country.

If values are implausible and fluctuate greatly, more frequent measurement is necessary. The values to be measured by the patient using CoaguChek systems and the laboratory control values that are required should be discussed with the doctor providing treatment.

3. What is the significance of a Quick value of e.g. 100%?

Significance: Value within the normal range.

An American scientist called Quick developed this method of determining the coagulation time in the 30s. The principle of Quick determination: Blood plasma or capillary blood is mixed with a coagulation reagent (= thromboplastin) and coagulated in a test tube. The time the sample needs to coagulate is measured. However, the result is not quoted in seconds but as a percentage of normal. The coagulation times are evaluated in a diagram: the longer the coagulation times, the lower the percentages.

For a person who does not take any oral anticoagulants (Vitamin K antagonists), the Quick value commonly lies between 70% and 130 %. A Quick value of e.g. 30% indicates the prolonged coagulation time. The lower the Quick value, the longer the blood takes to coagulate.

4. What is INR (International Normalised Ratio)

In the early 1980s, the World Health Organization recommended using INR instead of the Quick value as an indication of the coagulation determination.

INR is a standardization method that attempts to minimize differences between thromboplastin reagents through a calibration process in which all commercial thromboplastins are compared with an International Reference Preparation (IRP) maintained by the World Health Organization (WHO).
The INR method is not perfect in correcting for differences between different laboratories utilizing different thromboplastin reagents but it does reduce the variation between different laboratories and provides clinically useful results

5. How is the INR value calculated?

The INR value is also calculated from the patient's coagulation time. However, the ISI value is included in the calculation. The manufacturer calculates the ISI value (International Sensitivity Index) by comparison to an international standard thromboplastin. This way, the different sensitivities of reagents available in the market are compensated.

6. What is the formula for the calculation of the INR value?

First a patient's coagulation time is divided by a healthy individual's coagulation time (normal value). This ratio represents a patient's prolonged coagulation time compared to a healthy individual. However, since the sensitivities of the thromboplastins used are different (due to origin), the calculated sensitivity factor (= ISI) must be taken into account. The ISI is an exponent with which the ratio increases the effect.

MNPT (mean normal PT) is the geometric mean PT of minimum 20 healthy subjects in a defined laboratory population.

For example: The PT of a patient receiving oral anticoagulant is 64 seconds (= 18% Quick). The prothrombin time of a normal plasma is 22 seconds (= 100% Quick). The ISI of the thromboplastin used is 0.93. Substituting this value in the formula above gives the following INR:

(64) / (22) 0.93 = 2.7 INR

This signifies a coagulation time that is 2.7 times longer than the standard.

The longer the patient's coagulation time, the higher the INR.


7. How can comparability be brought about? What does ISI mean?

By using the International Sensitivity Index (ISI), the different INR can be compared. The results are then standardised. Each manufacturer matches its reagent to a standard reagent, the reference thromboplastin, proposed by the WHO. The ISI of one (the human origin one) of the reference thromboplastin is very close to 1. Each manufacturer determines the ISI value of its reagent lot by matching its thromboplastin production lot to a master lot which is measured against the WHO reference thromboplastin. The ISI value determined this way enters the calculation of INR as an exponent value depending on lot and method.

8. What does an INR value of 1 or 2 mean?

A person who does not take any anticoagulants (oral anticoagulants) has typically an INR value of 1. A measured INR value of 2 means that blood coagulation takes twice as long. Consequently, with an INR of 3, coagulation takes three times as long.

Conclusion: the higher the INR value, i.e. prolonged coagulation time, the more coagulation is inhibited.

9. What is the advantage of using the INR value?

INR is the international unit, recommended worldwide, for thromboplastin time (PT) determination. The WHO recommended the use of the INR instead of the Quick value as the unit for the measurement of coagulation right back in the early 1980s.

Reasons for using INR:

  • INR is comparable, i.e. coagulation measuring values can be compared despite different thromboplastins.
  • INR allows a standardisation of coagulation intensity for certain indication groups, independent of the thromboplastin and instrument used.
  • INR allows better patient therapy monitoring.
    The INR value is in inverse proportion to the Quick value.
10. What does therapeutic range interval mean?

The therapeutic standard range of the reagents is determined by the manufacturer. It is indicated in Quick percent and INR. The therapeutic range specified by the manufacturer is, generally speaking, the range within which the Quick/INR value of a patient should lie, who takes anticoagulants. A patient's individual conditions are not taken into account in the general therapeutic standard range.

11. Alcohol: Are Quick /INR values affected by the consumption of alcohol?

The following is true regarding the consumption of alcohol: larger quantities of alcohol have a coagulative effect. The chronic consumption of alcohol, however, has a reverse effect, i.e. blood coagulation is reduced, not to forget the risk of injury, if one "is no longer able to stay on one's feet". Thus it is all a question of the right balance!

12. Drugs: Are Quick/INR values affected by certain drugs?

A series of drugs have an increasing or inhibitory effect on anticoagulation. In any case, check the drug package insert under "interactions, side effects, intended use, contra-indications, special notes" whether implications with oral anticoagulants are indicated.

13. Which drugs intensify the effect of coumarin derivatives (Vitamin K antagonists)?
  • Pain killers (e.g. acetylsalicylic acid)
  • Antirheumatics (e.g. Phenylbutazon)
  • Antibiotics (e.g. Penicillin, Ampicillin, Tetracyclin)
  • Anticholesteremics (Statine, Fibrate)
  • Antiarrhythmic agents (e.g. Amiodaron, Chinidin)
  • Antidepressants (tricyclic antidepressants)
  • Diuretics (e.g. etacryn acid)
  • Hypoglycaemic drugs (e.g. sulfonyl carbamides)
  • Thyroid hormones

    This list is not comprehensive! Only the most common drug classes and drugs are indicated.
14. Which drugs reduce the effect of coumarin derivatives (VKAs)?

Reducing the effect means: thrombophilia increases, INR drops, Quick value rises:

  • Anti-epileptics (e.g. Carbamazepin, barbiturates)
  • Antimycotic drugs (e.g. Griseofulvin)
  • Antituberculous drugs (e.g. Rifampicin)
  • Vitamin preparations (vitamin K)
  • Laxatives are to be assessed with care because the effect on  anticoagulation is not predictable
This list is not comprehensive. Only the most common drug classes and drugs are indicated.