Factors That Can Affect The Laboratory Tests

Factors That Can Affect The Laboratory Tests

Laboratory tests, in particular, blood work are the most commonly prescribed tests in the USA.

The most common of these factors are diet, starvation, exercise, posture, daily and seasonal variations, menstrual cycle and pregnancy.

factors influencing blood tests

Fig: Factors That Can Affect The Laboratory Tests

Can the test be done quickly or the patient should prepare in some way, for example, fasting? Is the patient taking any drugs which may affect the results, for example: taking antibiotics before the fecal antigen test for Helicobacter pylori? Does the test need to be performed at a certain period?


  • Measuring testosterone in the morning?
  • For antibodies after Seroconversion?
  • A test result that is returning from the lab will basically cover a threshold value or reference guide based on guidelines.
  • On paper, this result doesn’t represent the significance of the test, which is applicable only when all the factors have been considered for the patient.

Here’s the general overview of concepts to help clinicians to consider different factors which can influence the interpretation of results. If you don’t know how a test can be affected by specific factors identified by you, it is important to discuss it with the lab first.

Ask these questions to yourself before going for lab investigation

  • What is the purpose of requesting the test? Is it to investigate signs and symptoms?
  • Are you keeping track of the effect or disease of a medicine? Is it a screening test?
  • Will this test improve patient care?
  • Will I understand it easily?
  • What is the procedure to take the sample?
  • About the collection and transportation of the sample?
  • How easy is it to interpret?
  • How they will inform you about the result?
  • Are there any harms of doing the test?

Biological Changes

There are certain changes in lab test results which we can expect because of non-modifiable factors. Those factors are biological rhythms, age, and physiological changes in pregnancy.

One can control these factors, i.e. by choosing the right time of the day, month or year.

You may consider in the interpretation of test results, such as the variation of reference ranges or thresholds for clinical changes according to age, gender, and prenatal stage.

Aging process

Apart from increasing poly-pharmacy and co-morbidity, the aging process is widely responsible for the change in test results.

Older people are more likely to have test results that have abnormal reference range. Labs can offer reference ranges according to age. But in the case of other laboratory tests, the result out of range in an older patient should be interpreted with the overall clinical image in mind.

Assessing the magnitude and rate of change over time provides more details in several cases than interpreting the value of the result. The population range of tests can show many more changes than that for a patient, such as a liver enzyme or serum creatinine. The previous results of the patient are helpful.

Interpreting laboratory tests results

When interpreting laboratory test results, a common sign of the effect of sex, age is observed. Also, the other variables are ALP or serum alkaline phosphate, which is asking for liver function. The upper limit of reference can increase reportedly during puberty as it is the time of maximum remodeling of bones.

ALP levels fall to another upper limit after this period through younger life and, it then rises again, especially in females around pre-menopause, which reflect a rise in bone turnover. In serum ALP, the rise may take place in women in the later stage of pregnancy (because placenta produces ALP), and other times like in weeks after healing the fracture.

Another common example of changing lab values is lipids which changes with age. Total triglyceride and LDL cholesterol levels increase with age, until around 50 to 60 years of age in males and 60 to 70 years in females, i.e. when they start to decline in most numbers of adults.

Triglyceride levels may keep on rising in older females. Most labs don’t report ranges for lipid levels as it is known to be less clinically important than targets on the major cause of increased levels and risk of heart disease.

When we discuss interpreting lab results in an older patient, make sure age-appropriate range is used when available. Normal changes relevant to age, i.e. poor renal function, may help determine results out of reference range in the older patient. However, it is not only the age that is responsible for the abnormal result.

Biological Parameters

Certain lab parameters vary according to the time of day, month or week or year when sampling them. Hormone production (like testosterone, cortisol, etc.), body temperature, cardiac function, platelet, and cognitive function follow a circadian rhythm. To work for this effect, there are some laboratory tests recommendations at a specific period of the day.

For example:

  1. The sampling of testosterone must be done from 7 am to 10 am. This usually occurs because very high testosterone levels take place usually in the early morning and they are significantly lower (around 50%) in the evening, especially in males at a young age.
  2. Serum cortisol should be sampled for most purposes in the early morning as diurnal variation is marked, with levels at least 50% to 100% higher in the early morning than that in the late afternoon.
  3. Vitamin D testing can be effected depending on the time of the year. The most obvious example being winter, where it is the lowest.

Menstrual Cycle

Females in their menstrual cycle have expected rhythms of LH, FSH, progesterone, and estrogen every month. In a general setting, such kinds of hormone levels may require to investigate the conditions like amenorrhoea/oligo. The meaningfulness and interpretation of result depend not just on value, but on stage in a cycle when the hormone was taken.

LH levels in a menstrual cycle peak mid-cycle to deal with ovulation. FSH levels are also responsible for peak mid-cycle. The predominant estrogen in ovulating women (Oestradiol) is highest before ovulation, and it then reduces if fertilization doesn’t take place.

The N.B. investigation of oestradiol levels is not important in women who are going to take estrogen-based oral contraceptives as it suppresses the ovarian axis. The FSH and LH are suppressed in women who are going to take depot progesterone.

We all know that Progesterone levels are to peak in the next menstrual phase, after ovulation, to prepare endometrium. It is often important to measure the progesterone levels to establish if ovulation takes place, i.e. usually 7 days before the period of menstruation, i.e. around day 21 of the month if the woman has the 28-day cycle.

During menopause, FSH levels tend to rise and oestradiol levels reduce, but it is not always reliable to monitor hormone levels when it comes to predicting the day of menopause, as changes occur as per the ovarian activity.


During pregnancy, psychological changes result in variations in several lab parameters, including renal and liver function, blood volume, and hormone levels. For different stages of pregnancy, reference ranges come for some medical laboratory tests. However, those ranges are sometimes not as define well as the general range.

Along with it, pregnancy-based changes like changes in protein binding can affect the assays differently.

Example: Free hormone levels that can depend on the assay. Hence, care is the must when interpreting laboratory test results. Also, based on reference ranges and patients should contact the labs in case of any doubt. When performing lab investigation in a pregnant woman, note down the week of gestation. Here are some of the common examples of lab values which change with pregnancy –

Reference Range

A reference test is a numerical range of results which is derived strategically by testing healthy people. However, ‘healthy’ is not simple and it relies on different assumptions. Some factors to define a range simply with ‘perfectly healthy’ in many cases to make it unusable and unrealistic.

The range is also referring widely to the Gaussian distribution. Hence, 68% of values are based on the standard deviation of the mean value, 99.7% of values in three SDs, and 95% in two SDs. However, a lot of ranges lack in Gaussian distribution but the upper end is skewed in the upper end. The range can derive by turning the log of data, or just by considering the 2.5th and 97.5th percentiles in a population that is being studied.

The Variation of Laboratory tests ranges

For lab results, reference ranges usually cover two SDs from the mean value. It means one in 20 healthy people have a test result out of reference range. These ranges vary from labs to labs and they can change if new evidence is available. The lower and upper limits are not absolute in the range and they don’t define any ‘abnormal’ or ‘normal’ results. But there are certain points in which the possibility of the clinical situation may arise.

Some of the ‘reference ranges’ depend on the changes from international bodies to optimize the outcomes of the patient, instead of the statistical distribution of the population. For instance, the upper reference range is based on guidance in TSH of the early pregnancy stage. For serum uric acid in patients having treatment to lower uric acid, the limit depends on the guidelines of the European League against Rheumatism (EULAR). Interpretation of results out of the given range depends widely on the clinical background of patient, clinical questions and pattern of other abnormal ranges.

Individual Changes

Health status, diet, and lifestyle factors of the patients can have the pre-analytical influence of lab changes.

Nutrition and Diet

Calorie restriction, fasting, malnutrition, food exclusion diets, and dehydration are responsible to affect lab results. In some laboratory tests, their significance may depend on certain dietary factors. It is vital to ensuring sample gluten in the diet for several weeks before serology tests for celiac disease or fasting before checking the effect of an intervention in a patient with high triglyceride levels. Assessing diet factors in some cases can help interpret unexpected results, i.e. a vegan or vegetarian diet which may reduce vitamin B12 levels. Increasing ketone levels and a high protein diet can increase uric acid levels due to the low carb diet.


Up to 12 hours of fasting before laboratory test may help get the most accurate result in these laboratory tests, according to the clinical scenario, which can affect by taking certain foods –

  • Uric acid – Usually, there is no need for fasting for accurate results but recent dietary intake may help determine unexpected outcomes.
  • Glucose, only when indicated – These days, HbA1c for most patients is recommended as a first choice test for monitoring and diagnosis of type-2 diseases, and do not need fasting.
  • Triglycerides – Fasting is not necessary for most patients in lipid testing but it may recommend monitoring high triglyceride levels in individuals.
  • Creatinine – High amount of meat in a recent meal can affect serum creatinine test significantly and it must in mind when testing for monitoring eGFR5

Starvation and low caloric intake – It results in some changes to parameters like thyroid, glucose, renal function, liver function, and lipids. The levels of uric acid may increase due to ketonemia (which is the cause of lower clearance).

Dehydration – It can be the cause of potassium and sodium imbalances and it can also affect several other indicators like urea and creatinine, hematology and lipids, etc.

Malnutrition – It is having different effects on lab results, according to the nature of the nutrition status of a patient. Malnutrition is considered as a lack of energy and protein, without deficiencies of micro-nutrient. However, malnutrition is defined as over-nutrition, under-nutrition, or deficiency of nutrients. Malnutrition is known to be the cause of results like reduced ferritin folate as well as levels of vitamin B12.


On laboratory tests, the effect of alcohol may vary on the extent and duration of use. Transient (acute) effects of consumption of alcohol (i.e. within 2 to 4 hours) include increased plasma lactate and serum glucose, with lower excretion of uric acid because of taking hepatic gluconeogenesis. Here are the chronic effects of consuming alcohol on laboratory tests –

  • The levels of gamma-glutamyl transferase (GGT), as well as mean cell volume (MCV), are widely using to test for high alcohol consumption.
  • Alanine aminotransferase (ALT), aspartate aminotransferase (AST) and ALT/AST ratio
  • Ferritin and uric acid because of alcoholic hepatitis and fatty liver.
  • Creatine kinase because of alcoholic myopathy.
  • Other abnormalities like thrombocytopenia and anemia.

Consumption of alcohol can also play a vital role in the deficiencies of minerals and vitamins. The effect may occur due to the replacement of food consuming with alcohol and vitamins like lower levels of vitamin A, folate, vitamin B and calcium.


When we discuss lab parameters, there is no complete study on the effect of caffeine. It is having a short lifespan of 3 to 7 hours, but it varies among people. Consumption of caffeine can cause elevated levels of blood glucose levels. It can also affect investigations like metanephrines interpretation when it comes to investigating hypertension.

The timing of investigation in case of illness

The significance of the investigation relies on the time of the sample taking to the stage of the disease. The selection of the right investigation can also influence the stage of illness. For instance, the serology test may negative for syphilis in case the sample is taken after exposure too early.

There are different types of serology test which provide details regarding past or active infection. Acute illness may also affect the result of investigations.

For example, Ferritin is an acute protein phase and levels can increase by infection and inflammation, along with malignancy and chronic disease.


Exposure to nicotine and smoking can both have chronic and acute effects on investigations. Within 1 hour after smoking up to 5 cigarettes, serum/plasma concentrations in fatty acids, glycerol, adrenaline, cortisol, and aldosterone may raise. Chronic smokers may have significant rises in leukocyte, lipoproteins, heavy metals, hematocrit and tumor markers, and reduce in activities of enzymes.


On laboratory parameters, the effect of exercise depends on the health status of the patient, intake of water and food after or during exercise. Vigorous or extreme exercise in a person who is not familiar with this activity level may cause changes in some lab parameters. For instance, the most common cause of increasing CK (creatine kinase) levels is exercise. CK levels can elevate due to intense exercise for several days to a week.

Thyroid function is also supposed to change among people going through the high-intensity workout. For instance, FT4 and TSH levels can increase in anaerobic exercise but it reduces FT3. AST and ALT levels can also rise after a workout. After a workout, haematuria and transient proteinuria are also common but they resolve after a couple of days.

There are some other analytes which can raise by exercises, those are as follows:

  • Creatinine
  • Urea
  • Prothrombin time
  • Lactate dehydrogenase
  • D-dimer levels

The activated partial thromboplastin time (APTT) and fibrinogen can reduce. Most of those effects may transitory but it depends on the individual health factors.

Analytical Changes

These variations take place due to errors in test methods as well as equipment. It may cause a slight difference in analyte values every time of measurement. Lab equipment and modern test methods mean more accuracy in lab test results. Usually, variation in measurement should less than half the biological variation of the patient.

After collecting a sample at the practice, it is vital to know about the type of container as well as the medium of the sample for a specific test. It can also affect the lab results. For instance, a PCR testing swab should move in a dry tube or tube with a viral transport medium.

Blood testing is one of the least transparent and most misunderstood parts of medical science. We simplify that with easy to understand results and affordable testing you can do at home.

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