Electrolytes Part 2 PDF

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Summary

MLSC-3111, Clinical Biochemistry IITotal CO The reference interval for TCO2 is 22 – 30 mmol/L. TCO2 = (HCO3-) + carbonic acid + dissolved CO2. We measure TCO2 as part of the electrolytes panel. Although different, TCO2 and HCO should correlate. Exposure to air can cause loss of CO2 gas (up to 3 – 4 ...

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MLSC-3111, Clinical Biochemistry II Total CO2 -

The reference interval for TCO2 is 22 – 30 mmol/L. TCO2 = (HCO3-) + carbonic acid + dissolved CO2. We measure TCO2 as part of the electrolytes panel. Although different, TCO2 and HCO3 should correlate. Exposure to air can cause loss of CO2 gas (up to 3 – 4 mmol/L if prolonged). Samples must remain capped to avoid CO2 loss. CO2 and Cl- are inversely proportional, so losing CO2 to the atmosphere results in increased Cl- levels.

Bicarbonate -

Most of the TCO2 is bicarbonate. It acts as an important buffer in blood. Decreased TCO2 is seen in metabolic acidosis, diabetic ketoacidosis and salicylate toxicity. Increased TCO2 is seen in metabolic alkalosis, emphysema and severe vomiting. Methods of analysis include the CO2 ISE and Enzymatic TCO2 (V350). The presence of high triglyceride concentration may cause light scattering and cause a negative bias in some enzymatic assays (falsely low).

Samples above and IV -

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Effects of an IV on LYTES depend on the IV solution. The saline drip (NaCl) dissociates into Na and Cl in solution. The sodium in the IV is usually on par with physiological concentration, but the Cl usually causes falsely high Cl. Results in lowered potassium and TCO2. Dextrose drips will cause glucose to be falsely elevated and all LYTES to be falsely low. Sterile water will dilute the sample and cause everything to be falsely low. Total protein also diluted by IVs. Sampling can be done above an IV if the IV is turned off 5 minutes prior to collection, the first tube is discarded and documented.

Electrolyte Disorders Diabetes Insipidus -

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Due to a lack of ADH or the inability to respond to ADH, the patient will experience polydipsia and polyuria. Results in extreme water loss. Can be caused by pituitary gland destruction (central DI), the placental enzymes destroying maternal ADH (gestational ID), inability of the kidney to respond to ADH (nephrogenic ID) or damage to the thirst-regulating mechanism of the hypothalamus causing polydipsia (primary polydipsia DI). Lab tests include decreased urine Na and OSM (< 200 mosm/kg) and increase plasma Na+ and Cl-.

MLSC-3111, Clinical Biochemistry II

Syndrome of Inappropriate ADH (SIADH) -

Overproduction/uncontrolled release of ADH from the pituitary causes increased water retention. Caused by certain drugs, infections, head trauma or pulmonary disease. Causes an increase in plasma water volume and dilutes LYTES and urine. Lab results seen are decreased plasma Na, K+, Cl- and OSM as well as increased urine Na, K+, Cl- and OSM. Even though there is high ADH in SIADH, it cannot be measured because the ADH assay is not common and not many labs have this test available. A specialized lab would be needed.

Addison’s Disease -

Hypoaldosteronism (with decreased cortisol) from adrenal atrophy with failure to release hormones leading to decreased Na+ reabsorption. Causes include autoimmune disorders (mostly) and cancers. Lab results seen are decreased plasma Na, Cl- and OSM with increased plasma K+. Increased urine Na, Cl- and OSM with decreased urine K+ is also seen. Lack of aldosterone affects the secretion of K+ since the loss of Na+ is being compensated for with K+. Aldosterone may be measured directly since the assay is more commonly available than ADH assays.

Conn’s Disease -

Primary hyperaldosteronism caused by the increased secretion of aldosterone from the adrenal glands. Results in increased Na+ reabsorption in exchange for potassium. Plasma will be high in Na, Cl- and OSM while urine will be low is Na and OSM. The plasma potassium levels will be extremely low because the excess aldosterone absorbed too much Na+, resulting in a proportional excretion of a lot of potassium. Results in high urine potassium and heart arrythmia.

The Anion Gap -

Equals the difference between routinely measured cations and routinely measured anions. Ions are divided into four groups; measured cations, measured anions, unmeasured cations, and unmeasured anions. Measured ions are the LYTES panel electrolytes (Na+, K+, Cl- and HCO3-) Unmeasured ions are ions that can be measured but aren’t part of the routine LYTES panel (Ca+, Mg+, sulfates, phosphates, etc.).

MLSC-3111, Clinical Biochemistry II -

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The Gamble Gram, a graph depicting the distributional relationship between anions and cations between the ICF, ECF and Interstitial fluid. There are always more unmeasured anions than unmeasured cations in the body. Under normal conditions, sodium, and potassium accounts for 95% of the total cations in the body. Cl- and HCO3- account for 85% of total anions. There are more measured cations than measured anions. An anion gap within the normal range demonstrates that plasma concentrations of cations are appropriately balanced by anions. Anion gap concentrations outside of the normal reference range provide a valuable tool used to indicate presence of electrolyte imbalances, evaluate acid base disorders and detect disorders like multiple myeloma. Formula used to calculate the anion gap is Na+ - (CL- + TCO2). Normal anion gap range is 6 – 18 mmol/L. K+ concentration is small and prone to error, so a simpler calculation can be done by omitting K+ from the equation. Also saves money from not running K+ assay. However, the math is the main issue as some labs run K+ in the LYTES panel anyways and adds K+ into their equation with an adjusted reference range.

Anion Gap Significance -

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Increased anion gap can be due to an increase in unmeasured anions or a decrease in unmeasured cations. Decreased anion gap can be due to a decrease in unmeasured anions or an increase in unmeasured cations. Metabolic acidosis causes production of acidic metabolites which in turn will release unmeasured anions to increase the overall anion gap value. (increased production of unmeasured anions and increased anionic proteins.) Causes relating to decreased cations are simply due to them being low. Hypoalbuminemia will cause the retention of HCO3- and Cl- in the ECF, resulting in a decreased anion gap. The anion gap aides in determining the cause of acidosis. Acidosis can be caused by too much acid produced, not enough acid eliminated or not enough base present to buffer the acid. Metabolic acidosis can be divided into 2 groups: high and normal anion gap MA. High AG metabolic acidosis is likely due to increased acid production. Normal AG metabolic acidosis is most likely due to a loss of plasma bicarbonate that is accompanied with hyperchloremia....