Assessing Possible Manipulation of the Urine Sample

The purpose of urine drug testing in clinical practice (Table V-7), where the majority of patients are not going to tamper with their urine sample, is to enhance patient care [62]. However, certain things can be done to improve the reliability of the results. Random collection is preferred, so the patient is not told in advance of the request for the urine sample. This can help to prevent the minority of patients who might tamper with their sample from being prepared with adulterants or substituted specimens.

In all situations, some control of sample integrity is desirable, and it is imperative where the consequences of incorrect results have far-reaching implications [61].

Table V-7. Possible manipulations of urine and how they are detected. Ultimately manipulated urine is unusable. It causes false negative as well as false positive results

Type of manipulation Substances, methods used

Characteristical changes


Addition of base

Addition of acids

Addition of other substances

Can occur in vivo or in vitro

Bleaches like acetic acid or Domestos® are added to the sample.

Acids like acetic acid or citric acid are added to the sample. Soap or kitchen salt is added to the sample.

Urine is very light. The temperature is under 32 °C. The specific weight is less than 1.01 g/ml. Creatinine content is less than 30mg/dl.

pH value is more than 8. Chloride scent may be present.

Marked pH value changes of >8

Urine may have flakes. Specific weight is greater than 1.035 g/ml.

Unobserved urine collection is usually acceptable, but observed collection may be necessary in certain situations or with high-risk patients. Ideally, the collection facility should not contain a basin with running water in order to reduce potential for specimen dilution, and blue pigment should be added to the toilet water. An unusually hot or cold specimen, small sample volume, or unusual color should raise concerns. Although urine specimens will cool to room temperature, the temperature of a urine sample within 4min of voiding should fall within the range of 90 °F-100 °F, which can be checked at the time of collection by a temperature strip built into the urine collection container. Urinary pH should remain within the range of 4.5-8.0, and urinary creatinine concentration should be greater than 20mg/dL A value less than 20 mg/dL is considered dilute and a value less than 5 mg/dL is not consistent with human urine [71]. Urinary creatinine measurement is an automated, inexpensive, and well-characterized method to test specimen validity. The color of a urine specimen is related to the concentration of its constituents [66]. Urine may be colored as a result of endogenous/exogenous substances derived from food pigments, medications, or disease states that produce excessive analyses. It can appear colorless as a result of excess hydration because of diet, medical condition, or water intake. In the absence of underlying renal pathology, patients who repeatedly provide diluted urine samples should be advised to decrease water intake prior to testing and to provide samples in the early morning. Any results outside of these ranges should be discussed with the patient and/or laboratory, as necessary [62].

The test results indicate whether the urine sample has been chemically manipulated before undergoing the drug test. Thinning of the urine sample is most likely the most frequently used form of urine manipulation. Specific weight and creatinine are significant parameters in case of a suspicious in vivo or in vitro thinning of the urine sample. Selective urine test strips are available, which can assess whether urine has been manipulated. The following parameters are being measured:

pH value - Any adding of acids or basic solution will be identified.

Nitrate - Nitrates in urine with gram-negative bacteria will be converted to nitrates. Nitrates can, if applied, falsify the test results.

Specific weight - A thinning of the urine sample can be identified.

Creatinine - Aside from in-vitro thinning, in-vivo thinning can also be recognized.

Glutaraldehyde - This is found in various disinfection solutions, it can falsify the test and will be detected with the strip. Bleach can affect the results if added to urine.

Pyridine chlorochromate - A chemical that will falsify the test

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