Introduction

The lipid profile routinely performed to assess cardiovascular risk involves the measurement/calculation of serum/plasma levels of total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG) and non-HDL cholesterol (non-HDL-C), although LDL-C level is still the most important factor in both the diagnosis and monitoring of the course and treatment of lipid disorders and the prediction of cardiovascular incidents [1–4]. It is worth emphasising, however, that since 2021 non-HDL-C has been treated as equivalent to LDL-C in the assessment of the lipid profile [5]. Similarly, there is no longer any doubt that it would be optimal to assess the number of atherogenic lipoprotein particles (rather than the mass of their components). Determination of apolipoprotein B (apoB) is still not a regular part of the lipid profile.

The results of lipid profile determinations indirectly, and approximately, reflect the content of the individual lipoproteins in the blood. Of particular importance in the laboratory assessment of lipid metabolism and the risk of progression of atherosclerosis is the quantitative measurement of atherogenic lipoproteins, i.e. LDL, lipoprotein(a) [Lp(a)], chylomicron (CM) remnants and very low-density lipoprotein (VLDL) remnants [2, 3]. This is why lipid profile, which usually applies only to LDL, should be augmented, whenever possible, by measurement of Lp(a), as well as assessment of triglyceride-rich lipoproteins (CM and VLDL) and their remnants.

Lipoproteins are a family of macromolecular structures composed of an ‘envelope’, containing phospholipids and free cholesterol, and a core composed of TG and cholesterol esters. The lipid part is bound to specific proteins – apolipoproteins (apo), which determine the physical and biological properties of lipoproteins. Lipids and proteins are not covalently attached to each other. The structure of lipoproteins is maintained primarily by hydrophobic interactions between the nonpolar components of lipids and proteins. The classification of lipoproteins reflects both their particle size and density in the aqueous plasma environment, as well as their apolipoprotein content (Figure 1). Triglyceride-rich CM and VLDL, as well as CM and VLDL remnants have a density of less than 1.006 g/ml. LDL, HDL and Lp(a) are lipoproteins with a density of over 1.006 g/ml.

Figure 1

Plasma lipoprotein particles size and density with the cholesterol they contain as a marker of their plasma levels

The lipid transport system involving lipoproteins has two main functions:

Dietary TGs are hydrolysed in the intestine into free fatty acids (FFA), mono- and diglycerides, absorbed together with exogenous cholesterol by enterocytes, where the CM that transport them are formed, and subsequently enter the circulation via the lymphatic system. Lipoprotein lipase (LPL) bound to the capillary endothelium of the adipose tissue and muscles hydrolyses the TG they contain into glycerol and FFA, with the formation of CM remnants. HDL particles are formed in the liver and intestine, and in the course of CM and VLDL degradation, from their superficial phospholipids and free cholesterol. The free cholesterol is taken up from peripheral cells (including macrophages in the vascular wall) by nascent-HDL and HDL3 subfraction, with the involvement of ATP-binding cassette transporter A-I (ABCA1) which binds to apolipoprotein A-I (apoA-I) and is then esterified by plasma lecithin-cholesterol acyltransferase (LCAT). Cholesterol esters are transported by mature HDL2 bound by the SR-B1 receptor in hepatocytes, where they are used for bile acid synthesis. This is a so-called direct mechanism of reverse cholesterol transport. By an indirect mechanism, cholesteryl ester transfer protein (CETP) transfers cholesterol esters from HDL to apoB-containing lipoproteins with simultaneous exchange for TG. Lipoproteins containing apoB are taken up by the liver via LDL receptors (LDLR), as well as LDL receptor-like protein (LRP) and other membrane receptors. TG hydrolysis in HDL2 by hepatic lipase (HL) leads to the formation of HDL3 (Figure 2).

Figure 2

Lipoprotein metabolism

ABC A1 – ATP-binding cassette transporter A1, CETP – cholesterol ester transporter protein, EL – endothelial lipase, HL – hepatic lipase, LCAT – lecithin cholesterol acyltransferase, LPL – lipoprotein lipase, PLTP – phospholipid transport protein, TG – triglycerides.

Currently available analytical methods provide only indirect, approximate insight into the transformation of both cholesterol and TG, or the metabolism and functions of lipoproteins. In clinical practice, the diagnostics of lipid metabolism disorders is a part of the assessment and control of the risk of atherosclerosis and atherosclerotic cardiovascular disease (ASCVD). Hence, the main goal of laboratory work-up for dyslipidaemia, defined as a condition in which blood lipid and lipoprotein levels are outside the desirable range, is aimed at measuring the amount of atherogenic lipoproteins in the blood. The methodological approach to lipoprotein measurement is varied at present. Their level in the blood can be determined directly as the number of particles (LDL-P, HDL-P, Lp(a)-P) or their concentration [Lp(a)] or assessed indirectly by determining the concentration of the components of individual lipoproteins - cholesterol or apolipoprotein (apoB, apoA-I).

Organisation of the guidelines

The members of the Steering Committee who prepared these guidelines were selected by the Polish Society of Laboratory Diagnostics (PSLD) and the Polish Lipid Association (PoLA) as experts in the diagnosis and treatment of lipid metabolism disorders. The Steering Committee has carried out a detailed review of the published scientific evidence on the management of dyslipidemia, including its diagnosis, treatment, monitoring, and prevention, as well as a critical evaluation of the diagnostic and therapeutic procedures, including risk-benefit assessment. Every chapter is summarised in a box containing clear and easy to understand recommendations, highlighting the information that needs to be remembered and the key points of the recommendation. This document is a supplemented version of the guidelines first published in this form in 2020 [6, 7].

The experts from the teams that developed and peer-reviewed the guidelines completed the conflict-of-interest forms with regard to all relationships which might be perceived as actual or potential sources of conflicts of interest. The final version of the document is published in the Diagnostyka Laboratoryjna (Journal of Laboratory Diagnostics), indicated by PSLD and Archives of Medical Science (indicated by PoLA).

Laboratory diagnosticians and physicians of various specialties who deal with patients with lipid disorders are encouraged to fully consider these guidelines when conducting clinical assessments, as well as defining and implementing medical prevention, diagnosis, or treatment strategies. Nevertheless, the guidelines in no way absolve physicians from individual responsibility for making correct and accurate decisions, considering the patient’s health status, and in consultation with the patient and, if necessary, with his/her caregiver. Healthcare professionals are responsible for verification of policies and regulations pertaining to medicines and devices in effect at the time of their prescription and/or use.

Pre-analytical considerations

Tests comprising lipid profile, both standard (TC, HDL-C, LDL-C, non-HDL TG) and additional measurements of apoB and Lp(a), are performed in serum or plasma. The general approach to the pre-analytical phase is based on the assumption that lipid profile should be assessed in the setting of everyday activity and diet, and people are not required to fast for about 16 h [8, 9]. Therefore, blood samples for these tests do not need to be collected when fasting [10]. This approach is also recommended in the 2019 position statement of the European Atherosclerosis Society (EAS) (2019) and the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) of 2016, which was based on data indicating that a slight postprandial increase in the TG level (to 27 mg/dl (0.3 mmol/l) does not result in significant changes in other measurements, or in the lipid profile assessment compared to the testing of fasting samples. Small differences in the interpretation of results pertain to TG and non-HDL-C. It is recommended that lipid profile be repeated in the fasting sample if the non-fasting TG level is > 400 mg/dl (4.5 mmol/l) [2, 10].

The levels of individual lipid profile components are characterised by intra-individual variability of 5–10% for TC and > 20% for TG. In addition to genetically determined mechanisms of lipid metabolism regulation, variability in TC levels is also caused by environmental factors such as physical activity, diet, smoking or long-term alcohol abuse, while variability in TG levels is affected by diet, including carbohydrate and alcohol intake, and physical activity. Changes in lipid profile occur during pregnancy, especially in the third trimester, and include predominantly an increase in the TG level (observed increase of up to 250%), TC and Lp(a) levels, and to a lesser degree the LDL-C (usually not more than 30%) and HDL-C levels [11]. Seasonal variation is also observed, with increases in TC and TG levels during winter [9, 4, 12]. The TC and LDL-C levels may be lower for several weeks after a CV event and in the course of chronic inflammation, e.g. rheumatic disease (lipid paradox), as well as in the elderly, especially over 75 years old [13, 14].

Due to the fact that cholesterol and TG are components of macromolecular lipoproteins, wearing a constricting band for ≥ 3 min or remaining in the standing position for more than 30 min before blood sampling may cause a 10–12% increase in their levels as a result of the so-called haemocentration effect. Serum levels of TC, HDL-C, LDL-C and TG are approximately 3% higher than the plasma levels. Serum or plasma samples may be stored at a refrigeration temperature (+4°C) for up to 4 days. Longer storage requires freezing at –70°C.

Triglycerides

Triglycerides (TG), triacylglycerols, glycerol and esters of long-chain fatty acids are the primary components of the adipose cells and the main source of energy for the body. TG are transported in the blood by chylomicrons formed in enterocytes, very low-density lipoproteins synthesised in the liver which, together with their metabolic products i.e. remnants, are referred to as triglyceride-rich lipoproteins (TRL). CM secretion is largely regulated by food intake, whereas VLDL secretion is controlled by insulin. TRL levels are also associated with a postprandial increase in TG level and approximately 80% of this increase is attributable to TG contained in TRL remnants.

TRL metabolism involves lipolysis of TG contained in CM and VLDL particles, catalysed by circulating and tissue lipoprotein lipases (LPLs), resulting in a reduction in TG concentrations in the blood. The second process is the exchange of TG and cholesterol esters between TRL and HDL/LDL via the cholesterol esters (CE) transfer protein (CETP). In effect, the resulting TRL remnants have smaller particle sizes, are depleted in TG, and enriched in CE, and are also depleted in apoC-III and enriched in apoE. TRL remnants are a heterogeneous group of lipoproteins, with variable density and particle size (Figure 3).

Figure 3

TRL particle size and density

VLDL remnants are in the IDL (intermediate density lipoprotein) and VLDL density classes, while chylomicron remnants (CM remnants) fall into the CM, VLDL and IDL density classes.

Given the intense development of triglyceride-lowering medicines, it is worth looking at the function of apoC-III, a protein encoded in humans by the APOC3 gene. ApoC-III inhibits lipoprotein lipase and hepatic lipase, creating conditions for inhibition of TRL uptake by the liver. Increased levels of apoC-III induce the development of hypertriglyceridemia, and its overexpression contributes to the development of atherosclerosis. Available data also suggest an intracellular role for apoC-III in promoting the assembly and secretion of triglyceride-rich VLDL particles from liver cells in hyperlipidaemia [15].

TRL remnant particles are removed from the blood through the uptake pathway by hepatocytes, via the receptor for low-density lipoproteins (LDLR), the LDL receptor-like protein (LRP) and heparan sulphate proteoglycans (HSPG) that bind apoE particles. The clearance of CM remnants is mediated by LRP or HSPG, whereas VLDL remnants are mainly removed via LRP. CM and VLDL remnants compete for the same uptake pathway involving apoE. Individuals with the apoE2 isoform were found to have reduced clearance of TRL remnants.

Smaller, highly modified TRL remnant particles are involved in atherogenesis. This is determined by both their small size and their high content of cholesterol and apoE. Remnant particles readily penetrate the endothelium (transcytosis) into the arterial walls, accumulate in the subendothelial space where they are bound by proteoglycans and are readily, without prior modification, taken up by LDLR-mediated macrophages with foam cell formation. FFA, released from local lipolysis, induce inflammation, platelet activation and oxidative stress, leading, in addition to endothelial dysfunction, to the initiation and progression of atherosclerotic lesions (Figure 4).

Figure 4

TRL remnants and atherosclerosis

Higher TG levels in blood, correlating with the accumulation of TRL and their remnants, is a cardiovascular risk factor independent of LDL-C levels, referred to as residual [16, 17]. This suggests a different approach to atherogenic dyslipidaemia defined as hypertriglyceridaemia with reduced HDL-C concentrations and increased content of small dense LDL. Hypertriglyceridaemia also increases the risk of cardiovascular disease in individuals with target LDL-C levels (even by more than 30%) [18]. Genome-wide association studies have revealed that susceptibility to ischaemic heart disease is associated with the presence of genes involved in TG metabolism [19]. Similarly, Mendelian randomisation studies indicate a causal relationship between TG metabolism and the risk of atherosclerosis and its complications, including ischaemic heart disease, increased in hypertriglyceridaemia by 33% [20].

Total cholesterol

Cholesterol is one of the best-known lipids, which results, among others, from its direct connection with the development of atherosclerosis. Cholesterol in the body comes from the diet, as well as biosynthesis occurring in the majority of cells, primarily in hepatocytes and enterocytes.

In the blood, it is transported primarily in LDL, and only about 30% of its circulating pool is found in HDL, VLDL, CM and VLDL remnants. Thus, determination of the TC concentration indirectly reflects an approximate plasma LDL level. From a clinical point of view, the TC level is currently used only to assess the severity of hypercholesterolaemia, in the absence of an LDL-C measurement result, and to calculate LDL-C and non-HDL-C concentrations [1, 28]. In cardiovascular risk, stratification scales such as SCORE2, SCORE2-OP, SCORE2-Diabetes TC has recently been replaced by non-HDL-C [29, 30].

HDL cholesterol

High density lipoproteins (HDL), unlike other lipoproteins, are characterised by a low lipid and high protein content. HDL transport about 25% of the cholesterol present in the blood, and its content in the particles of these lipoproteins varies considerably. Therefore, plasma HDL-C level provides indirect and inaccurate information on HDL content in the blood. Nevertheless, HDL-C measurement remains a basic test for the assessment of HDL content in the blood, as methods of direct measurement of the number of HDL particles (HDL-P), and their individual subfraction (measured with e.g. nuclear magnetic resonance spectrometry, ion mobility spectrometry, electro-phoretic techniques) are not available in routine laboratory diagnostics and do not provide sufficient new data to be recommended. It appears that the best method (not available in practice) to assess HDL functionalities is to assess their cholesterol efflux capacity (CEC). This is an in vitro test that measures the ability of HDL to promote cholesterol removal from cholesterol donor cells such as macrophages. CEC is a predictor of cardiovascular risk independent of HDL-C concentration [34, 35].

From a practical point of view, HDL-C level is not currently recommended as a treatment target or predictor of cardiovascular risk or for use in monitoring the treatment of lipid disorders [4, 5]. HDL-C level is used in the calculation of LDL-C and non-HDL-C levels.

Methods of determination

The HDL-C level is measured in serum or plasma. Previously used methods required ultracentrifugation or, more commonly, precipitation to isolate the HDL fractions. The reference method for the determination of HDL-C was a combination of ultracentrifugation and chemical precipitation to separate HDL from other lipoprotein classes containing apoB [36]. In the late 1990s, direct (homogeneous) assays for the determination of HDL-C were introduced into medical diagnostic laboratories [3]. Direct assay, without precipitation of LDL and VLDL, is possible thanks to the use of a detergent which dissolves HDL and adsorptively blocks access of enzymes (esterase and cholesterol oxidase) to cholesterol in VLDL and LDL particles.

New-generation homogeneous methods (several types) are widely available, and ready-to-use reagents allow full automation of HDL-C measurement in the primary serum/plasma sample [3]. Direct methods are well standardised (for samples from healthy individuals) and ensure sufficient accuracy of measurement. Measurement bias, if any, is mainly due to the matrix effect, e.g. in dyslipidaemia. In measuring the HDL-C level, neither precipitation nor current direct methods differentiate between the HDL subclasses discussed below. According to the NCEP recommendations, the total allowable error for HDL-C direct method measurement is ±13% for normolipidemic samples and between –20% to +36% for dyslipidemic samples. The majority of inaccurate results are seen at HDL-C levels < 40 mg/dl (< 1.0 mmol/l) [36]. In COBJwDL surveys, the applicable error limit is ±15% and this value is also recommended by PSLD/PoLA (2024).

HDL are a heterogeneous group of small discoid and spherical particles, differing in density (1.063–1.21 g/ml), size (7.6–10.6 nm) and electrophoretic mobility, as well as apolipoprotein and lipid content [37, 38]. Apolipoprotein A-I (apoA-I) is the major protein component of the HDL particle, accounting for about 70% of the protein content and playing a significant role in HDL function and biogenesis [39]. HDL can be fractionated using different techniques depending on their physicochemical properties and composition [40] (Figure 5). Two fractions are obtained by sequential ultracentrifugation: HDL2, a fraction of large light particles, rich in lipids, with a density of 1.063–1.125 g/ml, and HDL3, a fraction of small, dense particles, rich in proteins, with a density of 1.125–1.21 g/ml. HDL2 and HDL3 particles are not homogeneous fractions. They can be separated by gradient electrophoresis on a polyacrylamide gel into five subfractions of decreasing size: HDL2b, HDL2a, HDL3a, HDL3b and HDL3c. Using a bidirectional electrophoresis method that allows separation according to charge and particle size, more than 10 HDL subfractions ca be obtained. To standardise nomenclature, it has recently been proposed to divide HDL into 5 subclasses according to their physical and chemical properties [41]: 1) very large HDL, 2) large HDL, 3) medium HDL, 4) small HDL, and very small HDL.

Figure 5

HDL subpopulations and measurement techniques

The analytical methods/measurement techniques listed in Figure 5 allow for direct determination of HDL particles in serum/plasma (HDL-P), as well as differentiation of their subfractions, which is made possible by a certain functional characteristic [40, 42]. Due to the different results obtained for the different HDL subfractions with regard to their predictive properties (most studies indicate that small dense HDL is a proatherogenic fraction), they are assessed mainly for research purposes [43–45]. As mentioned above, perhaps research on CEC will change our ability to assess the function of HDL particles.

Recommendations

Currently, the measurement of HDL-P requires standardisation of methods and the definition of desirable (target) values, which makes it impossible to perform these tests routinely.

There is no direct evidence at present of the usefulness of HDL subfraction measurement in cardiovascular risk assessment, and cholesterol efflux capacity (CEC) assessment is not available in clinical practice.

Dysfunctional HDL

The anti-atherosclerotic effect of HDL is primarily related to its involvement in reverse cholesterol transport, but also to its anti-inflammatory, antioxidant, anti-apoptotic, anticoagulant, cytoprotective and vasoconstrictive activity. Altered HDL properties, in particular the development of dysfunctional HDL particles, constitute a factor for an increased cardiovascular risk (there has been a worldwide debate for many years whether this is the correct name) [46–49]. It is caused primarily by inflammation, but also by oxidative stress and glycation. Increased expression of myeloperoxidase (MPO, E.C. 1.11.1.7) plays an important role [50]. MPO catalyses the modification of apoA-I and consequently inhibits ABCA1-dependent reverse transport of cholesterol, contributing to foam cell formation and the development of fatty infiltration of a blood vessel (Figure 6 A). Inflammation also induces HDL transformation consisting in the:

Figure 6

Dysfunctional HDL particles: A – HDL modified by myeloperoxidase, B – inflammatory HDL

SAA – serum amyloid A, PON-1 – paraoxonase-1, GPx – glutathione peroxidase, RCT – reverse cholesterol transport, ABCA1 – ATP-binding membrane cassette transporter A1.

PON-1 (aryldialkylphosphatase, E.C. 3.1.8.1) plays a special role in reducing CV risk. It is an enzyme that hydrolyses toxic organophosphorus compounds, phospholipid peroxides and cholesterol ester hydroperoxides [51]. The role of PON-1 is to protect the LDL fraction from oxidative modification, preventing the formation of atherogenic oxidised LDL particles (oxLDL) [52, 53].

HDL-C level does not provide information on HDL functionality. To date, methods for the direct determination of dysfunctional HDL have not been developed for routine use. Knowing the mechanisms of their formation, it is possible to try to predict this process in inflammation diagnosed and monitored using standard markers: C-reactive protein (CRP) and interleukin 6 (IL-6), as well as MPO and PON-1, which are directly related to the dysfunctionality of these lipoproteins. From a practical point of view, in the absence of a gold standard (reproducible, simple, and cheap) for assessment of HDL functionality, determination of dysfunctional HDLs has no clinical relevance. On the other hand, knowledge of this topic is crucial, because already in primary prevention in obese patients, in smokers, or in secondary prevention and/or in chronic kidney disease – the majority of HDL particles may be dysfunctional or even reveal similar atherogenic properties as LDL particles. Such a situation can be observed in patients with high HDL-C levels (> 100 mg/dl/2.6 mmol/l), which not only does not reduce, but even increases the risk of cardiovascular death, irrespective of the cause or risk of cancer [54–56]. The explanation may be precisely that most HDL particles in these individuals have impaired function (are dysfunctional).

LDL cholesterol

Low-density lipoproteins (LDL) transport approximately 70% of the cholesterol present in the blood. Cholesterol and its esters account for 40–50% of the LDL particle mass. Due to the crucial role of LDL in atherogenesis, the LDL-C level, which indirectly reflects the LDL content in the blood, serves as a primary lipid cardiovascular risk factor and its specific values represent a goal for lipid-lowering therapy. In view of the low detectability of hypercholesterolemia in Poland (only 2/10 people know their cholesterol level, and only one in five is on a therapeutic target [57, 58]), lipid profile testing, including LDL-C level measurement, should be promoted. As blood samples for lipid profile do not need to be collected while fasting, the testing is more widely available and calculation/determination of the LDL-C level is easier [2–4, 10].

Non-HDL cholesterol

Non-HDL cholesterol (non-HDL-C) is an integrated indicator of the blood content (mass) of all lipoproteins that, according to current knowledge, are associated with the initiation and progression of atherosclerosis. These include the apoB-containing particles LDL, VLDL, IDL, CM, TRL remnants and Lp(a). Determination of non-HDL-C level is very important for cardiovascular risk assessment and, as of 2021 (Guidelines of PoLA and five other scientific societies [5]), is recommended as a permanent element of the lipid profile because it complements LDL-C by allowing the assessment of residual risk and, based on available studies, is more predictive of cardiovascular risk than LDL-C levels and most often equally predictive as apoB measurement reflecting the number of atherogenic lipoprotein particles [65, 66]. Non-HDL-cholesterol is now also a recommended parameter in the assessment of cardiovascular risk in SCORE2 and SCORE2-OP and, according to recent Polish recommendations, is the basis for the diagnosis of metabolic syndrome [67].

The non-HDL-C level is calculated according to the following formula:

The calculation of non-HDL-C is more reliable than the calculation of LDL-C [66, 68]. Nevertheless, as with the use of other formulas, the accuracy of non-HDL-C calculation depends on the biological and analytical variability of TC and HDL-C concentrations. However, the biological variability of HDL-C levels is much lower than that of other lipid parameters, especially TG. In addition, HDL-C concentrations are much lower than the TC levels, which minimises their effect on changes in calculated non-HDL-C concentrations.

Apolipoprotein B

Apolipoprotein B (apoB), which is a structural component of all lipoproteins except for HDL, exists in two isoforms: apoB100 (MM 550 kD), synthesised in hepatocytes and present in VLDL, IDL and LDL, and its fragment, apoB48 (MM 265 kD), synthesised in enterocytes and present in CM and their remnants [10, 69].

Small dense low-density lipoprotein

Small dense LDL (sdLDL) are a lipoprotein fraction with low density, but more atherogenic properties than the so-called “large buoyant LDL” (lbLDL), due to their small size and significant modification of their particles, mainly by oxidation, and easy penetration through endothelial cells (transcytosis) into the subendothelial space of the arterial membrane, where they are extensively bound by macrophage scavenger receptors and absorbed leading to the formation of foam cells.

Despite the documented role of sdLDL in atherogenesis and the recognition of their increased content (phenotype B) as a feature of atherogenic dyslipidaemia, the role of this lipoprotein fraction as an independent risk factor for ASCVD is still under investigation and debate, and it is not assessed for diagnostic purposes. Determination of sdLDL by lipoprotein subfraction analysis techniques (ultracentrifugation, NMR spectroscopy, etc.) is not available for routine laboratory operations. A new perspective is offered by the homogeneous methods now available for the determination of sdLDL cholesterol (sdLDL-C) and the formulas developed for calculating its concentration. The newly derived equations depend on two components, LDL-C as defined by the Sampson-NIH equation and the factor of interaction between LDL-C and the natural logarithm of the TG level. In 2021, Sampson et al. proposed an equation to assess large buoyant LDL:

in order to calculate the sdLDL-C level on this basis:

Subsequently, based on data from the MESA (Multi-Ethnic Study of Atherosclerosis) study, the authors have showed that in a multivariate analysis, standardising the results against other known ASCVD risk factors, the estimated sdLDL-C level had the strongest association with ASCVD compared with other lipid parameters [71]. Ongoing studies on the diagnostic characteristics of this parameter represent the next step before potentially adding measurements/calculations of sdLDL-C to the lipid profile.

Lipoprotein (a)

The apo(a) molecule is characterised by marked amino acid sequence homology with plasminogen; it contains the protease domain and the so-called Kringle IV and V domains. The molecular mass of Lp(a) shows high interindividual variability, dependent on the number of repeats of the Kringle IV type 2 (KIV-2) domain, which may range from 3 to 40 and is genetically determined by the number of tandem repeats of the genome sequence in the LPA gene. This genetically determined size of the Lp(a) particle is inversely proportional to the rate of its synthesis mainly in the liver and the serum/plasma concentration – particles of a lower mass (lower number of KIV-2 repeats) are present in higher concentrations, while particles of a higher mass (high number of KIV-2 repeats) are present in lower concentrations [10, 72].

Lp(a) level is a risk factor independent of LDL-C for ASCVD, calcified aortic stenosis, peripheral artery disease or ischaemic stroke. In Poland, Lp(a) levels are measured too infrequently and knowledge of them is inadequate. Therefore, every effort should be made to change this as soon as possible, as increased Lp(a) levels occur quite frequently. According to available data, up to 30% of patients with familial hypercholesterolaemia and/or acute coronary syndrome may have Lp(a) concentrations above 125 nmol/l (50 mg/dl), with often desirable LDL-C levels. In addition, there are emerging opportunities to reduce Lp(a) levels pharmacologically using PCSK9 inhibitors, inclisiran, or ultimately medicines dedicated to lowering Lp(a) levels, such as pelacarsen or olpasiran, which will further increase the need for assays [73–75].

Lipid profile – laboratory report

Lipid profile includes a battery of blood serum or plasma tests and calculations aimed at identification of dyslipidemia as a cardiovascular risk factor, defining the recommendations and treatment monitoring, including:

In addition to the results of measurements and calculations, a lipid profile laboratory report (Table IX), should include information on how the LDL-C level was determined (calculated/determined), as well as the target (desirable) and alarming levels of the assayed analytes. When severe dyslipidaemia is suspected, it should also include information on the need to seek urgent medical attention if the LDL-C level indicates a possible diagnosis of heterozygous (> 190 mg/dl; 4.9 mmol/l) or homozygous (> 500 mg/dl; 13.0 mmol/l) familial hypercholesterolaemia (FH), if the Lp(a) level > 125 nmol/l (> 50 mg/dl) indicates a high risk of cardiovascular incidents, or the TG level > 880 mg/dl (10.0 mmol/l) indicates, in addition to increased cardiovascular risk, a high risk of acute pancreatitis or, in the case of some typical symptoms, a risk of familial chylomicronaemia syndrome (FCS).

Information provided on a lipid profile order form on whether a patient is overweight/obese and/or suffers from diabetes and whether he/she is receiving a lipid-lowering therapy is helpful in laboratory interpretation and authorisation of obtained results.