D1 Basic biochemistry

Topic

Basic biochemical testing measures metabolite and ion concentrations in the blood to assess metabolic status as well as renal and hepatic function. Many assays employ enzymatic reactions that generate colored products; the light absorption of these products depends on the analyte concentration. By applying the Beer-Lambert law, this absorption allows for the quantitative determination of the concentration present in the sample.

One indicator derived from these measurements is the anion gap, calculated by subtracting the sum of chloride and bicarbonate from the sodium level. When it rises above the reference range, it indicates the presence of unmeasured anions and points toward acidosis caused by the accumulation of substances such as lactate, ketoacids, products retained due to renal failure, salicylates, or metabolites of toxic alcohols. If metabolic acidosis is present but the anion gap remains normal, the cause may be bicarbonate loss, as seen in diarrhea or certain renal tubular disorders.

The osmolar gap provides complementary information. It is obtained by comparing directly measured plasma osmolality with the osmolality calculated from known concentrations of sodium, glucose, and urea nitrogen. Substances not included in the calculation—such as methanol or ethylene glycol—can raise measured osmolality and create a difference between the two values. Thus, the combination of an elevated osmolar gap and high-anion-gap acidosis can point to the presence of these toxic alcohols.

Plasma osmolality reflects the total concentration of osmotically active particles in the blood. Its calculated value depends primarily on sodium, with minor contributions from glucose and urea nitrogen. When sodium levels rise, osmolality tends to increase, causing water to shift from cells into the plasma and resulting in cellular dehydration. When sodium levels fall, the reverse occurs, and cells take up water. This swelling is particularly dangerous in the brain, where it can lead to cerebral edema. Renal function is assessed indirectly via plasma creatinine. Since this substance is filtered by the kidneys and only a relatively small amount is additionally secreted by the tubules, its concentration provides information on renal filtration capacity. The glomerular filtration rate is estimated based on creatinine levels and patient characteristics such as age and sex. A decrease in this rate indicates impaired renal function and allows chronic kidney disease to be classified into different severity categories—information that is crucial for clinical monitoring and for adjusting medications that rely on renal clearance.

Hepatic assessment utilizes parameters including the aminotransferases ALT and AST. These enzymes are normally found intracellularly and rise in the bloodstream in the event of hepatocellular injury. Significant elevations may occur, for instance, in cases of viral hepatitis, drug toxicity, or ischemic damage. The relative pattern of these two enzymes offers further insight: ALT predominance is often observed in viral hepatitis, whereas a markedly elevated AST/ALT ratio points toward alcohol-related liver damage. Changes over time also help track the progression of the injury.

These parameters do not function in isolation but are interpreted in conjunction with one another. The anion gap and osmolar gap aid in investigating the causes of metabolic acidosis; sodium and osmolality allow for the assessment of water distribution and the risk of cellular abnormalities; creatinine and the estimated glomerular filtration rate provide information on renal functional capacity; and aminotransferases enable the detection and characterization of hepatocellular injury.

Thus, using a blood sample and calculated ratios between its components, basic biochemical analysis allows for the reconstruction of fundamental aspects of the body's metabolic balance. Combining directly measured concentrations with derived indicators transforms a collection of metabolites, electrolytes, and enzymes into tools for detecting metabolic disturbances, assessing renal function, and identifying liver damage, while also guiding diagnostic and therapeutic decisions.

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