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is saliva good as blood in disease diagnosis (testing)? why people prefer to use blood sample over saliva sample?

 


Blood has long been considered the ‘go to’ medium for a range of diagnostic tests, with saliva testing downplayed because of issues with sensitivity, specificity and reproducibility. However, saliva testing has advanced significantly over the years, overcoming these problems to become an excellent choice for various analytes – particularly hormones.

A Measure of Bioavailability

Saliva is a wonderful choice for hormone analysis, where the issue is not necessarily how much is being produced – which can be established by blood testing – but how much is available for the body to use. Typically, hormones such as estrogen, progesterone, testosterone and cortisol are bound to carrier proteins in the bloodstream, with just a small proportion unbound and available to exert its effect. In contrast, saliva contains only the free, bioactive hormones, which is the fraction that is of interest to clinicians and wellness practitioners. Measuring the bioavailability of a hormone is therefore critical for effective patient management in a number of conditions because it clearly shows any imbalance. As a result, saliva testing is increasingly being used for adrenal checks – cortisol and the testosterone precursor dehydroepiandrosterone (DHEA) – as well as for monitoring the sex hormones estrogen, progesterone and testosterone.

More than a Question of Convenience

Blood and urine testing have their place for hormone analysis, but saliva testing has a number of advantages in terms of accuracy and convenience. For example, the traditional way of monitoring hormones is a 24-hour urine collection. While this is entirely valid, it is not very convenient for the patient, and will only determine the total amount of a hormone produced; it will not provide the hour by hour profile throughout the day. Similarly, when clinicians need to look at diurnal variations in cortisol, they ideally require morning, noon, afternoon and night-time samples. The invasive nature of blood sampling presents a major challenge for this type of testing, requiring a compliant patient and a trained individual to perform the sampling. This makes repeated sampling during the day unviable. On top of that, many people have a needle phobia, potentially causing a stress response that could skew cortisol results. Some will even actively avoid attending appointments for a blood test, however important it may be. This is where saliva testing comes into its own, as samples can be quickly, easily and conveniently taken as often as necessary in the patient’s own home, with no risk to the individual.

Simplified Sample Handling

Taking a sample is just the first step. It still has to be transported to the laboratory for analysis in a manner that prevents any deterioration of the matrix or analyte, which could compromise the end result. Blood collection tubes are often only designed and validated for short-term storage and transport – just a few hours, rather than days, depending on the analyte. This can present a major issue when samples are collected at locations a long distance from the laboratory, as transport by road, rail or air could take several days. There is a real risk of sample degradation over time, with the integrity of the result being called into question. Saliva, on the other hand, is easy to collect and transport, with a variety of sample collection tubes on the market for added flexibility.

Choosing the Right Assay

Saliva-based diagnostic testing has historically been hindered by a lack of commercially available kits. As a result, laboratories wanting to test saliva had to develop and validate their own assays in house. Today, the situation has changed, with an increasing number of manufacturers recognizing the value of saliva testing, and launching a variety of assays for this matrix. With a choice of tests on the market, it is essential to look carefully at how each one has been validated, checking the specifications to ensure that it is fit for the intended purpose, and will deliver meaningful results for the requesting clinician. The cheapest solution is not always the best, as there may be a trade-off between cost and, for example, sensitivity. Choosing an assay with demonstrated sensitivity, accuracy, precision, reliability and reproducibility – such as those from Tecan – can help to ensure reliable diagnoses and increase demand for this type of testing, as well as benefitting patient management.

Looking Beyond the Assay Kit

The long-term performance of any diagnostic or screening test is dependent on ongoing quality control (QC) testing to safeguard the accuracy and reliability of analytical results. Using high-quality test kits and validated internal QC testing regimes is obviously essential for any lab, but it doesn’t stop there. Some diagnostic test manufacturers, including Tecan, go a step further by offering laboratories an independent proficiency testing program. Two or three times a year, samples are sent to participating laboratories for analysis with the specified assay, and the results are reported directly to the program coordinator. The results are collated and shared anonymously with all the participants. This allows each laboratory to see whether its results are comparable to those of other facilities, giving them confidence that the assay is being performed correctly.

Another key consideration is the availability of reference ranges; without reference ranges, there is no way of relating an analytical result to control samples and normal values, rendering it clinically valueless. Interpreting the result also depends on the availability of additional information about the patient, and an understanding of how these values may differ according to, for example, whether the patient is male or female, or when the sample was taken. In addition, the female sex hormones – estrogen and progesterone – will change depending on where the patient is in their monthly cycle, and differ between premenopausal, menopausal and postmenopausal women. The availability of relevant reference ranges and technical support is therefore essential to help the laboratory provide clinicians with meaningful diagnostic information.

Saliva Testing in the Wellness Arena

Hormones are implicated in many aspects of an individual’s overall health and wellness, including stress, mood, memory, energy, menstrual cycles, weight loss or gain, sleep and libido. Any imbalance can have a negative effect on a person’s wellbeing, often leading the sufferer to consult healthcare professionals in the wellness space – wellness practitioners, naturopaths, nutritionists, chiropractors, physicians in functional or integrative medicine space – when conventional medicine has not been able to provide answers.

Saliva testing offers wellness practitioners a straightforward way to obtain baseline hormone levels, acting as a starting point for further investigations. Some of the most commonly requested tests are for the stress hormone cortisol, and the sex hormones – estradiol, progesterone, DHEA and testosterone. The diurnal cortisol test is a convenient means of monitoring the body’s stress level over the course of the day and, along with DHEA, helps to provide an indication of any imbalance in adrenal function. Similarly, determining the levels of sex hormones will help individuals seeking an optimal hormone balance. While the overall cause of a hormonal imbalance may be complex, such tests open the door to finding solutions.

Conclusion

Saliva testing is one of the best ways of looking at many analytes and has a wonderful future, offering a non-invasive method of measuring the bioactive levels of hormones and other compounds. Today, almost any of the proteins that can be tested for in blood – for example, enzymes, cytokines and antibodies – can also be screened in saliva, at sensitivities that could only be dreamt of just a few years ago. Saliva-based detection levels are now into the low picogram range, with new assays offering novel analytes and lower detection limits continually under development. Saliva testing has truly come of age

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